Back to Calculation Tips & Tricks
- even numbers 2 through selected 2-digit evens
- digits of square of repeating ones
- consecutive odds
- consecutive between 2 numbers
- sequence from 1 to selected 2-digit number
- sequence from 1 to selected 1-digit number and back
- sequences in the 10's
- sequences in the 20's
- sequences in the 30's
- sequences in the 40's
- sequences in the 50's
- sequences in the 60's
- sequences in the 70's
- sequences in the 80's
- series of doubles
- series of quadruples
- series of 10 numbers
- 1's repeating, divide by 9, subtract 21
- 8's repeating, divide by 9, subtract 10
- squares of two numbers
- reversing/adding/subtracting 3-digit numbers
- finding 2.5 percent
- finding 5 percent
- finding 15 percent
- finding 20 percent
- finding 25 percent
- finding 33 1/3 percent
- finding 40 percent
- finding 45 percent
- finding 55 percent
- finding 60 percent
- finding 70 percent
- finding 75 percent
a series of quadruples
- Have a friend choose a single digit number. (No restrictions for experts.)
- Ask your friend to jot down a series of quadruples (where the next term is always
four times the preceding one), and tell you only the last term.
- Ask your friend to add up all these terms.
- You will give the answer before he or she can finish: The sum of all the terms
of this series will be four times the last term minus the first term, divided
- If the number selected is 5:
- The series jotted down is: 5, 20, 80, 320, 1280.
- Four times the last term (1280) minus the first (5):
4000 + 800 + 320 - 5 = 5120 - 5 = 5115
Divide by 3: 5115/3 = 1705
- So the sum of the quadruples from 5 through 1280 is 1705.
See the pattern? Here's one for the experts:
- The number selected is 32:
- The series jotted down is: 32, 128, 512, 2048.
- Four times the last term (2048) minus the first (32):
8000 + 160 + 32 - 32 = 8,160
Divide by 3: 8160/3 = 2720.
- So the sum of the quadruples from 32 through 2048 is 2720.
Practice multiplying from left to right and dividing by 3. With practice you will
be an expert quad adder.
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3 June 1996
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