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A number ends in 5. Drop that 5, double the rest, add 1. What does this give for 85?

A16
B18
C19
D17
Answer & Solution
Correct answer: D. 17
1. Remove the last digit 5 from 85 to leave 8. 2. Double it: 8 x 2 = 16, then add 1 to get 17. 3. This equals 85 / 5 = 17; 16 forgets the added 1. _Source: Wikipedia, Divisibility rule, section Divisibility by 5_
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