Skip to main content

2013 AMC 10A Problem 13

Problem 13 of 25IntermediateNumber TheoryProblem-Solving Techniques

How many three-digit numbers are not divisible by 5,5, have digits that sum to less than 20,20, and have the first digit equal to the third digit?

Answer choices

Show solution

Solution

Note that for the number to not be divisible by 5,5, the units digits cannot be either 00 or 5.5. Let xx be the hundreds and units digit and yy be the tens digit. Then we want 2x+y<20. 2x + y \lt 20. Casing on the 88 possible values of x,x, we get: If xx is 1,2,3,1, 2, 3, or 4,4, then yy can be anything since y<10.y \lt 10. If x=6,x = 6, then y<8,y \lt 8, which gives us 88 solutions. If x=7,x = 7, then y<6,y \lt 6, which gives us 66 solutions. If x=8,x = 8, then y<4,y \lt 4, which gives us 44 solutions. If x=9,x = 9, then y<2,y \lt 2, which gives us 22 solutions. This gives us a total of 4⋅10+8+6+4+2=60 4 \cdot 10 + 8 + 6 + 4 + 2 = 60 solutions. Thus, B is the correct answer.
AoPS wiki

Tagged: digits · casework

More practice