I am a two-digit multiple of 7. If you add my digits together, the sum is a prime number. What number am I?
21 (or 35, 42, 56, 70, 77, 91)
Multiples of 7 include 21 (2+1=3, prime), 35 (3+5=8, composite), 42 (4+2=6), 56 (5+6=11, prime), etc. Any of these valid multiples work as a solution.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Medium. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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