2010 AMC 12B Problem 19

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19.

A high school basketball game between the Raiders and the 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

Answer: E
Concepts:geometric sequencearithmetic sequencebounding to limit cases
Difficulty rating: 2180
Solution:

Let the Raiders score a,ar,ar2,ar3a, ar, ar^2, ar^3 (increasing geometric, r>1r\gt1) and the Wildcats a,a+d,a+2d,a+3da, a+d, a+2d, a+3d (increasing arithmetic), tied in the first quarter at a.a.

Write r=m/nr=m/n in lowest terms, with m>n.m\gt n. Since ar3ar^3 is an integer, n3a;n^3\mid a; put a=An3.a=A n^3. The Raiders' total is R=A(n3+mn2+m2n+m3). R=A(n^3+mn^2+m^2n+m^3). Since m3<R100,m^3\lt R\le100, we have m4.m\le4. The pair (4,3)(4,3) already makes the parenthesized sum 175,175, so the only possible coprime pairs (m,n)(m,n) are (2,1),(3,1),(3,2),(4,1).(2,1),(3,1),(3,2),(4,1).

The corresponding base values of R/AR/A are 15,40,65,85.15,40,65,85. For (4,1)(4,1) and (3,2),(3,2), the bound forces A=1,A=1, but R1=4a+6dR-1=4a+6d gives a nonintegral d.d. For (3,1),(3,1), it would give 36A1=6d,36A-1=6d, which is impossible modulo 6.6.

For (m,n)=(2,1),(m,n)=(2,1), we have R=15AR=15A and a=A,a=A, so 11A1=6d.11A-1=6d. Thus A5(mod6),A\equiv5\pmod6, and R100R\le100 forces A=5.A=5. Then d=9,d=9, giving Raiders scores 5,10,20,405,10,20,40 and Wildcats scores 5,14,23,32.5,14,23,32. The Raiders win 7575 to 74.74.

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

Thus, the correct answer is E.

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