In a country there are only four types of coins, of denominations , , and dollars respectively. In how many different ways can one pay exactly dollars using these coins?
Solution
Answer:
Note that and have a common factor of . Also, and are greater than and respectively. We call the coins with denominations and 'bad coins'.
Since , we must use bad coins such that . The smallest such is , and hence we must have (because the next would be , and the total amount will surely exceed ).
We note that . To pay exactly , we may start with coins with denomination , and add units of . This can be done by using a coin (which contributes units), a coin (which contributes units), or by replacing a coin by a coin (which contributes unit). Hence the problem is reduced to solving the equation in nonnegative integers. Since each choice of for which gives a unique choice of , we only have to solve the inequality in nonnegative integers. We find that there are solutions as shown below, and so the answer is .
| Value of | Possible corresponding values of |
|---|---|