2 x 3 x 7 = 42
2 x 2 x 2 x 3 x 3 = 72
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You can elimate the even numbers 72 and 42 right away because the only even prime number is 2. Using the divisibility rules, you can eliminate 87; 8 + 7 = 15, which is divisible by 3, which means 87 is divisible by 3. 59 is the prime number.
6 is not a prime number but it is a factor of 72
92 - 18 = 72 72 - 16 = 56 56 - 14 = 42 42 - 12 = 30 30 - 10 = 20 20 is the next number in the sequence.
42 = 2 x 3 x 7 72 = 23 x 32 84 = 22 x 3 x 7
30 + 42 = 72