Yes, and for small values of X, 70/X is more accurate, too. 72 has the advantage of being a multiple of 2, 3, 4, 6, 8, 9, and 12, whereas 70 is a multiple of 2, 5, 7, 10, and 14 -- knowing both of these gives you a whole-number answer for any growth rate under 15% except 11% (correct answer is 6.64) and 13% (correct answer is 5.67).
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Using 70/X will also work just fine.
Yes, and for small values of X, 70/X is more accurate, too. 72 has the advantage of being a multiple of 2, 3, 4, 6, 8, 9, and 12, whereas 70 is a multiple of 2, 5, 7, 10, and 14 -- knowing both of these gives you a whole-number answer for any growth rate under 15% except 11% (correct answer is 6.64) and 13% (correct answer is 5.67).