Arturo O. answered 10/06/16
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m = number of marbles = 42
s = number of stones
m/s = 6/7 ⇒ s = 7m/6 = (7*42) / 6 = 49
There are 49 stones. Test the solution:
m/s = 42/49 = (6*7) / (7*7) = 6/7 (good)