The way I do it is that I add the numbers of the ratio first (7+2=9) - this gives me a numnber of "units"
now, each Unit will be the number divided by this sum: 9.
so 45/9=5
so each unit is "5".
and the ratio will be the multiplication of this,
7*5=35
2*5=10
so the result will be 35:10
