1968 AHSME Problems/Problem 19
Problem
Let be the number of ways dollars can be changed into dimes and quarters, with at least one of each coin being used. Then equals:
Solution
Because we have to use at least one dime and one quarter, this part of the selection is forced. Thus, the number of ways to partition in the desired manner is equal to the number of ways to partition into quarters and dimes without restriction. However, is not a multiple of , so we must use dimes until we reach a multiple of . We first reach such a multiple at , and, because we have had no freedom up to this point, the number of ways to partition into dimes and quarters will yield our desired answer. can be partitioned into 37 quarters, and we can replace quarters 2 at a time with 5 dimes to partition it differently. We can replace anywhere from 0 to 18 pairs of quarters with dimes in this manner, giving us different partitions, which is answer choice .
See also
1968 AHSC (Problems • Answer Key • Resources) | ||
Preceded by Problem 18 |
Followed by Problem 20 | |
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