Question

Ben is 140cm tall. Mike is 1090mm tall. Who is taller? How much taller?

Answer 1

1cm = 10cm
140cm=1400mm So Ben taller

Difference is bigger minus smaller
1400-1090=310mm :)
               (=31cm)

Answer 2

1090mm into m = 109cm 
$\left[\begin{array}{ccc}1& ^{3} 4& ^{10} 0\\1&0&9\\\\&3&1\end{array}\right]$ so Ben is taller by 31cm