They form a proportion when both of the ratios are equivalent.
An example: 3/15 and 1/5 is a proportion because the are equivalent.
The reason why they are equivalent is because if you cross multiply meaning doing 3*5 which equals 15 and 15*1 which equals 15 so basically you multiply the numerator by the other fractions denominator and do the same for the other side and see if the numbers are the same if they are not the same they are not a proportion if they are the same they are a proportion.
