2015 AMC 10A Problems/Problem 13
Problem 13
Ashraful has 12 coins, each of which is a 5-cent coin or a 10-cent coin. There are exactly 17 different values that can be obtained as combinations of one or more of her coins. How many 10-cent coins does Ashraful have?
Solution 1
Let Claudia have 
5-cent coins and 
10-cent coins. It is easily observed that any multiple of 
between and 
inclusive can be obtained by a combination of coins. Thus, 
combinations can be made, so 
. But the answer is not 
because we are asked for the number of 10-cent coins, which is 
Solution 2
Since the coins are 5-cent and 10-cent, all possible values that can be made will be multiples of 
To have exactly 
different multiples of 
we will need to make up to 
cents. If all twelve coins were 5-cent coins, we will have 
cents possible. Each trade of a 5-cent coin for a 10-cent coin will gain cents, and as we need to gain 
cents, the answer is
For small scale substitutions of 5 cent for 10 cent, like in this problem, it is quite fast, however, if the problem is a little bit more complex, knowing that you need 85 cents, explained above, it is also possible to use a system of equations. It would be
![\[5x+10y=85\]](http://latex.artofproblemsolving.com/f/e/e/feef9fa2a66020eae20f4850306fbfcfc511a060.png)
![\[x+y=12\]](http://latex.artofproblemsolving.com/a/5/d/a5d38bfc59e64867544c7fedd40f4be483be5b00.png)
Solving this (5-cent coins) 
and 
(10-cent coins) 
, so again the answer is
Video Solution
~savannahsolver
See Also
| 2015 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 12 |
Followed by Problem 14 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
| All AMC 10 Problems and Solutions | ||
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