Date: 02/12/2002 at 22:20:04 From: Levi Avalos Subject: Multiplication My son brought home his homework, and there are two questions we don't understand: 1. Can you use the same multiplication fact to find 4x50 and 5x40? Explain. What is a multiplication fact? 2. Can you use the same multiplication fact to find 4x50 and 3x80? Explain. I don't understand what they are asking. Can you please help? Thanks, Levi Avalos and his mother, Michelle
Date: 02/13/2002 at 22:15:29 From: Doctor Jodi Subject: Re: Multiplication Hi Michelle and Levi! These are some good questions. First of all, a multiplication fact is part of the times table (like 2x3 = 6). You know that 2x3 = 3x2, right? (In general, you can switch the order of multiplication.) For your first question, I would think about 4x50 as 4 x 5 x 10 (since 50 = 5x10) And I would think about 5x40 as 5 x 4 x 10. Which multiplication facts do both of these use? Can you come up with a way to do this multiplication in your head? For your second question, we could think about 3x80 as 3 x 8 x 10. Hmm. That doesn't look very much like what we had for 4 x 50 (4 x 5 x 10). Can we make these look more alike? A little bit: We could write 8 as 4 x 2, so then we'd have 3 x 4 x 2 x 10 and 4 x 5 x 10. These look a little similar - they both have a 4 and a 10 - but they're mostly different. Can you figure out how to do 3 x 80 in your head? Rearranging the factors (the numbers in these multiplication facts) helps sometimes. Have fun multiplying, and feel free to write back if you have more questions! - Doctor Jodi, The Math Forum http://mathforum.org/dr.math/
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