2010 AMC 10B Problem 24

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Concepts:geometric sequencearithmetic sequenceDiophantine Equation

Difficulty rating: 2180

24.

A high school basketball game between the Raiders and Wildcats was tied at the end of the first quarter. The number of points scored by the Raiders in each of the four quarters formed an increasing geometric sequence, and the number of points scored by the Wildcats in each of the four quarters formed an increasing arithmetic sequence. At the end of the fourth quarter, the Raiders had won by one point. Neither team scored more than 100100 points. What was the total number of points scored by the two teams in the first half?

3030

3131

3232

3333

3434

Solution:

Let the first-quarter score for each team be a.a. Let the Raiders have common ratio rr and let the Wildcats have common difference d.d.

Write r=m/nr=m/n in lowest terms. Since the Raiders' four quarter scores are integers, a=n3Aa=n^3A for some positive integer AA, and their total is A(n3+n2m+nm2+m3)100A(n^3+n^2m+nm^2+m^3)\le100. Thus the only possible ratios are 32,2,3,4\dfrac{3}{2},2,3,4.

For r=32r=\dfrac{3}{2} or r=4r=4, the only possible Raiders totals give Wildcats totals that are not of the form 4a+6d4a+6d. For r=3r=3, the equation 40a=4a+6d+140a=4a+6d+1 is impossible modulo 66.

For r=2r=2, the equation is 15a=4a+6d+115a=4a+6d+1, so 11a=6d+111a=6d+1. The only positive solution with total at most 100100 is a=5,d=9a=5,d=9.

The first-half total is 5+10+5+14=34.5+10+5+14=34.

Thus, E is the correct answer.

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