See also the
Dr. Math FAQ:
order of operations
3D and higher
Browse Middle School Equations
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
Solving simple linear equations.
- Rearranging an Equation [3/25/1996]
Rearrange the formula E=1/2mv to make v the subject.
- Rearranging an Equation [05/01/1999]
Solve for x in terms of y: y = (900x)/(2+x).
- The Secret to Mathematics? [09/27/2004]
I understand how to divide fractions--invert and multiply--but what if
the dividend or divisor is a variable?
- Setting Up Algebraic Word Problems [08/04/2003]
If each photocopy of a manuscript costs 4 cents per page, what is the
cost, in cents, to reproduce x copies of an x-page manuscript?
- Setting up an Equation [05/25/1999]
When I add 4 to a number I get the same result as halving the number and
adding 10. What is the number?
- Setting Up Equations [11/26/1997]
An airplane travels eight times as fast as a car. The difference in their
speeds is 420 km/h. How fast is each vehicle travelling?
- Simple Equation: 3y + 2 over 4 equals 7 [10/17/2001]
I think you have to subtract 2 on both sides, but I'm lost.
- Simplify and Solve [01/15/1998]
Simplify one side of the equation. Then solve. .75y + y + 10 = 45
- Simplifying Answers [12/17/1994]
How do you answer questions like this? (p^2q^-4)^2 (p^3q^5)^-
- Simplifying Expressions [11/24/1997]
Solve: 2x - ( 3 - x ) = x - 7 .
- Simultaneous Equations [07/01/2002]
A man has a total of 20 dimes and quarters. If the dimes were quarters
and the quarters were dimes he would have 90 cents more than he has
now. How many of each coin does he have?
- Solutions to Equations [01/16/1997]
How do you make a table of values of x and y that make the equation 3/4y
= -x true?
- Solve for X [10/09/1999]
How can I solve equations like 4x - (1/2) = -3/2?
- Solving a Linear Equation in One Variable [02/27/1998]
Solve for q: -13 - q - 9 = 5q - 9 - q.
- Solving an Equation with Decimals [1/31/1996]
I can't figure out what 15.6 + z = 20.4 is. I need step-by-step help.
- Solving an Equation with Fractions [1/30/1996]
v = 6 - 5/4 v. I don't know if this question is asking 6 minus 1 1/4 or 6
* 1 1/4.
- Solving another Equation in One Variable [1/12/1996]
-2[3-2(x-1) + 5(2-x)] = -3[x+2(x-1)]
- Solving a System of Three Equations in Three Variables [06/23/2007]
How can I find values of x, y, and z that make all three of these
equations true: x + y + z = 10, x - 2y + z = -2, and x - y - z = -12?
- Solving Basic Equations with Variables [11/28/2004]
How do I solve an equation like 8x + 6 = 3x + 21?
- Solving Equations [6/28/1996]
Solving Systems of Equations: find k, if 7/x+y = k/x+2 = 11/2-y
- Solving Equations with Fractions in Them [12/06/2004]
How do you solve an equation like -5/7 n = 35?
- Solving Equations with Square Roots [03/28/2003]
When solving x + 1 = sqrt(x + 3), we can square both sides then solve
the equation; however, we get an extra solution that doesn't work. Why?
- Solving for a Variable in One Equation [3/18/1996]
I need to write the following in standard form and solve for the
indicated variable: (2r+3)(2r-1) = -(3r+1)
- Solving for One Uknown among Many: How and Why [09/20/2010]
A student wonders how to solve for an unknown that appears in the middle of several
others. Over the course of several back-and-forths with many examples, Doctor Ian
reveals the different scopes of what it means "to solve," introduces the concept of sets
of solutions, explains why you might want to isolate one term with respect to others,
shows how to do so, decodes the meaning of a string of variables -- and reaffirms
that, indeed, all the rules that the student has learned still work no matter the symbols.
- Solving for One Variable of Many [05/30/2002]
How do you solve a formula with many variables for one of them?
- Solving Linear Equations and the Scale Metaphor [08/26/2004]
When solving linear equations, why do we have to do the same operation
on both sides of the equal sign?
- Solving Linear Equations by Using Inverses [08/13/2003]
Solve for a in b = 5/7(a-8).
- Solving Pairs of Equations by Substitution [11/26/1997]
x + 3y = - 5; 3x - 2y = 7.
- Solving Problems With and Without Algebra [05/14/2002]
In a basketball game, Harold and Isaac scored a total of 19 points.
Isaac and Jacob scored a total of 14 points. Isaac scored as many
ponts as Harold and Jacob together. How many points did each player
score? And how can I solve this without algebra?
- Solving Simple Equations [02/23/1998]
Equations like 3(30+s) = 4(s+19) and others following the same form.
- Solving Systems of Linear Equations [02/09/1999]
Can you help me solve simultaneous equations?
- Solving Two Equations with Mixed Fractions [05/13/1998]
Steps to solving (2 2/3)x - 1 1/4 = 4 1/3 and (4 1/5)x - 2 = 3 1/3.
- Splitting Peanuts [08/03/2002]
On a camping trip, the family had brought many bags of peanuts for
- Step-by-step Solution of Equation in One Variable [05/13/1998]
Commutative, associative, distributive, and other laws of arithmetic
applied to solve w in the equation 2 - 2(w + 6) = 2(w + 5).
- Straight Line on the Graph [09/10/1998]
Let f(x) = ae^(kx) where a>0 and k are constants. Show that the graph of
y = ln(f(x)) is a straight line.
- Substituting Parameters for Numbers [11/29/2001]
I don't know what to do with both sides of the equation 17.20X/100 = X-
- Subtracting One Equation from Another [05/30/2001]
Why can you subtract one equation from another and get something
meaningful that you can use to solve the equations?
- The Sum of Two Numbers is 20... [11/03/1997]
Twice one number is 4 more than four times the other. Find the numbers.
- Switching Dollars and Cents [10/07/1997]
How do I find an equation?
- System Of Equations With Products Of Variables [10/29/2003]
I'm trying to solve a system of three equations and three variables
where the variables are multiplied together, such as 'xy' and 'yz'.
Substitution doesn't seem to be working very well - what's a good way
to get started?