Ends in 5, 10 Apart
Multiply two ...5 numbers that are 10 apart (35×45, 65×75).
35 × 45 = 1575
35 and 45 sit 5 either side of 40, so this is (40 − 5)(40 + 5).
That is the difference of squares: , or 1600 − 25.
A multiple of ten squared ends in 00, and 00 − 25 is why it always ends 75.
Your turn
Three to try in your head — tap what you get.
35 × 45
25 × 35
55 × 65
All 36 shortcuts: the Magic Tricks library.