Question

A teacher bought 7 boxes of pencils. They cost $12each. How can the teacher best use mental math tofind the total cost before tax?7 x 127x 127 x 127 (8+4)7 8 + 7 x47 (9 + 3)7 x 9 + 7 x37 (10 + 2)7 × 10 + 7 x 256 + 2863 +2170 + 14$84TA$84

Answer 1

I suppose the $12 not contains tax.

So the better way to make mental math is split the 12 into a sum and use the associative property od multiplication:

$\begin{gathered} 7\cdot12=7\cdot(10+2)=7\cdot10+7\cdot2=70+14=84 \\ \end{gathered}$

The answer is 84