What is 9 xx 7 = ?

Jun 29, 2018

63

Jun 29, 2018

A bit of a cheat approach

Explanation:

To multiply by 10 is more straightforward but this will produce an error. This we can compensate for.

$9 = \left(10 - 1\right)$

so $9 \times 7 \textcolor{w h i t e}{\text{d}} = \left(10 - 1\right) \times 7$

$\textcolor{w h i t e}{\text{d") (10xx7)-(1xx7)color(white)("ddd") =color(white)("ddd")70-7color(white)("ddd")=color(white)("ddd}} 63$