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Number theory Difficulty 4.7 AIME Prove it Hong Kong

In a country there are only four types of coins, of denominations 7474, 8787, 111111 and 124124 dollars respectively. In how many different ways can one pay exactly 20232023 dollars using these coins?

Solution

Answer: 1414

Note that 7474 and 111111 have a common factor of 3737. Also, 8787 and 124124 are 1313 greater than 7474 and 111111 respectively. We call the coins with denominations 8787 and 124124 'bad coins'.

Since 202325(mod37)2023 \equiv 25 \pmod{37}, we must use kk bad coins such that 13k25(mod37)13k \equiv 25 \pmod{37}. The smallest such kk is 1919, and hence we must have k=19k = 19 (because the next kk would be 19+3719 + 37, and the total amount will surely exceed 20232023).

We note that 2023=19×87+3702023 = 19 \times 87 + 370. To pay exactly 20232023, we may start with 1919 coins with denomination 8787, and add 1010 units of 3737. This can be done by using a 7474 coin (which contributes 22 units), a 111111 coin (which contributes 33 units), or by replacing a 8787 coin by a 124124 coin (which contributes 11 unit). Hence the problem is reduced to solving the equation 2a+3b+c=102a + 3b + c = 10 in nonnegative integers. Since each choice of (a,b)(a, b) for which 2a+3b102a + 3b \le 10 gives a unique choice of cc, we only have to solve the inequality 2a+3b102a + 3b \le 10 in nonnegative integers. We find that there are 1414 solutions as shown below, and so the answer is 1414.

Value of bbPossible corresponding values of aa
000,1,2,3,4,50, 1, 2, 3, 4, 5
110,1,2,30, 1, 2, 3
220,1,20, 1, 2
3300

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.