Brain Teasers & Mental Math
Mental math: what is 13 × 17? (multiply two numbers both in the teens)
Model answer
221.
Teens trick: (10 + a)(10 + b) = 100 + 10(a + b) + a·b. Here a = 3, b = 7: 100 + 10(10) + 21 = 100 + 100 + 21 =…
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