You will have to write a proportion. Then use common denominators to write equivalent ratios. For example;
What is the weight of a 3 Ib human brain in ounces?
Use a proportion to convert 3 Ibs to oz.
16 ounces ? oz
————— = —————
1pound 3 pounds
Use common denominators to write equivalent ratios.
16•3 ?
----- = --- 3 is a common denominator.
1•3 3
48 ?
--- = ---
3 3
Equivalent rates with the same denominators have the same nominators.
?= 48
The weight is 48 ounces.