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".