Answer:
10 feet. It doesn't look right, but in fact it is.
Step-by-step explanation:
Set up a proportion. Be sure and put the unknown in the numerator. That way you avoid cross multiplication
Let the building's height be x.
x/25 = 6/15 Multiply both sides by 25
25 * (x / 25) = 25*(6/15) Cancel what is common on the right.
x = 5(6/3) Divide 6/3
x = 5(2)
x = 10