When a photo is reduced or enlarged it's length to width ratio usually remains the same .aurelian wants to enlarge a 4 in by 6 in photo so that it has a width of 15 inches

Length:   4      2     10
Width:     6      3     15

You can tell because 15 isn't divisible by 6, but it is divisible by 3, which goes into 6.

Divide 6 by 2 to get 3.

To keep the ratio, we must also divide 4 by 2 to get 2.

Then, multiply 3 by 5 to get 15 for the width.

You must also multiply 2 by 5 to get the length of 10.

The picture's dimensions will be 10"x15".