How to scale down an image by aspect ratio exactly?

Question

I have an image that is (just for example, can be any size):

599 original width
599 original height

and a destination aspect ratio for example (can be any):

1:2

I want to scale down the image and make it exactly the aspect ratio, a simple multiplication gives only:

299.5 x 599

while I expect

299 x 598

How to get the expected result?

Sorry for my bad English, I find it hard for me to describe a mathematics question well.

Answer 1

You don't say what the class of problems you are working on, but one approach is to divide $599$ by $2$ and round down because you can't have fractional pixels. That gives the new dimension in that direction as $299$. Then multiply that by $2$ to get the size in the other direction, giving $598$. This gives the largest rectangle of proportion $1:2$ that fits in $599 \times 599$