a × b
The trick, step by step
Halve the even number: 16 becomes 8.
Double the other: 35 becomes 70.
Multiply the easy pair: 8 × 70 = 560, so 16 × 35 = 560.
If the new pair is still hard, do it again: 24 × 25 = 12 × 50 = 6 × 100 = 600.
given working answer taken away
Why it works
(a ÷ 2) × (b × 2) = a × b, because halving and doubling cancel out. Think of a rectangle 16 by 35 cut in half and put end to end: 8 by 70, the same area.
Practise this trick