Question

Find the Distance, d, across the lake

Answer 1

D=11.25
You can make a proportion of the bigger triangle to the smaller triangle 
8/15=6/d
cross multiply 
8d=90
d=11.25<span />

Answer 2

The answer is 11.25 ft. 
You have to use similar figures to work this out.
15 ft / 8 ft has to equal d ft  / 6 ft
So, you divide 15 by 8, which equals 1.875
You then times 1.875 by 6 = 11.25