Multiplication and division problems, sans calculator

[justin.lam06]

Hi Tommy - can you demo again how to do both a multiplication and division problem without a calculator? (I have the general gist, but would like more clarification about applying place value rules when adding.)

[tommywallach]

Hey Justin,

Place value is its own separate thing, I'm just talking about using groups of tens, fives, and ones.

31 * 22 =

10 * 22 = 220

so

30 * 22 = 660

so

31 * 22 = 682

Does that make sense? Take a group of 10, multiply the result by 3 (to get a group of 30), then add the remaining 22. That's all there is to it in multiplication. (For decimal place, just count off.)

For division, you use the equation:

x/y = Quotient + remainder/y

so 92/7 -->

7 goes into 92...13 times.

(Because 7*13 = 91)

The remainder is 1, so we're left with:

13 + 1/7

And if you know that 1/7 is about .14, you know you have 13.14.

Hope that helps!

-t