Vincent had 7/10 as many stamps as Isabelle. After Vincent bought another 30 stamps, he had 15 less stamps than Isabelle. How many stamps did Vincent have at first?

Answer 1

v = 7 i/10 so i = 10 v/7
v + 30 = i - 15

v + 30 = 10 v/7 - 15
45 =10 v/7 - 7 v/7 = 3 v/7
v = 15 * 7 = 105
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check
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then i = 10v/7 = 150
105/150 = 7/10?
7*15 = 105 Yes !
135 = 150 - 15 YES !!