4 by 6 inch photo is enlarged to a width of 15 inches. What's the new length?

First, you can write a proportion width/legth=width/length

4/6=15/x

Cross multiply (Multiply one denominator times the other numerator and vice versa)

4 (x) = 6 (15)

4x=90

divide both sides by 4

x=22.5.

The new length is 22.5