You will want to compare the width to the length, or 42 mm/ 74mm ...the units cancel, which is what you want for a ratio...and you usually want to simplify your ratio
If we have 300 animals total, and the ratio of cats to all animals is 7 : 15 or 7/15, we can multiply that fraction/ratio by the total, so 300 * 7 /15 = 20 * 7 = 140.
