A gallon holds 16 cups, so you can estimate the number of coins in a five-gallon water jug by first determining how many coins fit into one cup. Multiply the result by 16 to get the number of coins in one gallon. Finally, multiply by five.
Four cups fit into one quart. Four quarts fit into one gallon. Therefore, 16 cups make up one gallon.
The answer to the problem therefore only depends on the kind of coins used. Pennies, nickels, dimes, and quarters all have different sizes.
If the jug is already full of coins, the number of coins can be estimated by weighing the jug. Subtract the unfilled weight of the jug from the filled weight, and then divide that figure by the average weight of a coin. Be aware that different coins have different weights.