You need to understand what a ratio is. It is basically equivalent to a fraction. i.e. the ratio 2:3 can also be expressed as a fraction 2/3 (this is what I do for ratio questions, anyway). For your problem:
m : n = 2 : 5
Expressing them as fractions gives:
m/n = 2/5
And then you can sort that out and the answer pops out.