How to keep the original ratio when changing the width?

Question

Say I have a rectangle that is 390 x 200.

I want to change the width to 400.

What's the math to use so that the new height fits the original ratio?

Answer 1

Which is the width and which is the height? You have multiplied the width by $\frac {new\ width}{old\ width}$ so multiply the height by the same factor.