Wiskundetruuks
\eqalign{\frac{2}
{5} - \frac{1}
{4} = \frac{8}
{{20}} - \frac{5}
{{20}} = \frac{3}
{{20}}}
$
22 \times 31 = \left( {20 + 2} \right)\left( {30 + 1} \right) = 600 + 20 \cdot 1 + 2 \cdot 30 + 2 = 682
$
92 \times 98 = \left( {100 - 8} \right)\left( {100 - 2} \right) = 10000 - 1000 + 16 = 9016
$