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AmandaPanda11702
I got D. 7.5 because the sides are in a 4 to 1 ratio. The corresponding side of 40 on the smaller triangle is 10 and 40/10 is 4. to find the corresponding side of 30, do 30/4 and you get 7.5
The figures are similar, therefore they have corresponding proportional sides. So, match up the corresponding sides and set up a proportion.

40/30 = 10/x

cross multiply  and divide to get 7.5