Because you're only using a single decimal place I assume you're referring to 21.6 pounds by weight and not £21.60 by value.
Also, it's not clear which country you're referring to - the U.S.,...
A typical penny weighs 2.5 grams. 15lbs converts to 6,804 grams. 6,804 divided by 2.5 equals 2,722 pennies. So the 15lb. bag of pennies roughly equals $27.22
From my understandings, a pound of pennies is worth about 145 cents ($1.45), which is approximately a dollar and a half. But it also depends on how much the penny weighs itself. So if you are trying...