Is the following a Proportion

15/40 = 5/8

No, because 5/8=15/24 and 15/24≠15/40

15/40 = 5/8

cross multiply

(15) (8) = (40) (5)

120 = 200 ... incorrect ... so it is not a true proportion

little tip : proportions are nothing but equivalent fractions ... 15/40 is not equivalent to 5/8 ... now if it would have been 15/40 = 3/8, since these are equivalent fractions, this would have been a true proportion