Maths Olympiad Prep

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Algebra Difficulty 3.0 AMC 10/12 Prove it Canada

In a game, a player throws a ball at a target. If they hit the
target, then 7 points are added to their score. If they miss the target,
then 33 points are subtracted from
their score. A player’s score begins at 0, and it is possible for a
player to have a negative score.

What is Shane’s score after 66 throws if 44 of the throws are hits and 2 of the
throws are misses?
After exactly hh hits and 6 misses, Susan’s score is 59.
What is the value of hh?
After exactly 2020 throws, Souresh’s score is greater
than 85 and less than 105. If exactly mm of these throws are misses, determine
all possible values of the positive integer mm.

Solution

Each hit adds 7 points to Shane’s score, and so 4 hits adds 7×4=287\times4=28 points to his score.

For each miss, 3 points are subtracted from Shane’s score, and so 2
misses subtracts 3×2=63\times2=6 points
from his score.

A player’s score begins at 0, and so after these 6 throws, Shane’s score
is 286=2228-6=22.
Solution 1

After exactly hh hits and 6
misses, Susan’s score is given by the expression 7×h3×67\times h-3\times6 or 7h187h-18.

After these throws, Susan’s score is 59, and so 7h18=597h-18=59.

Solving this equation, we get 7h=59+187h=59+18 or 7h=777h=77, and so h=11h=11.

Solution 2

Susan’s 6 misses decrease her score by 3×6=183\times6=18 points.

Since her score is 59, then Susan must have scored 59+18=7759+18=77 points from hits.

Since each hit is worth 7 points, then the value of hh is 77÷7=1177\div7=11.
Solution 1

We begin by making an initial guess at the value of mm, and then systematically adjust the
value upward or downward as needed.

If we begin with m=3m=3 for example,
then the number of hits is 203=1720-3=17
and Souresh’s score is 7×173×3=1199=1107\times17-3\times3=119-9=110.

Since this score is greater than 105, then the value of mm is greater than 3 (more misses means
fewer hits and a lower score).

When m=4m=4, the number of hits is
204=1620-4=16 and Souresh’s score is
7×163×4=11212=1007\times16-3\times4=112-12=100.

Since this score is greater than 85 and less than 105, then 4 is a
possible value of mm.

When m=5m=5, the number of hits is
205=1520-5=15 and Souresh’s score is
7×153×5=10515=907\times15-3\times5=105-15=90.

This score is also greater than 85 and less than 105, and so 5 is a
possible value of mm.

When m=6m=6, the number of hits is 14
and Souresh’s score is 7×143×6=9818=807\times14-3\times6=98-18=80.

This score is less than 85 and so 6 is not a possible value of mm.

Continuing to increase the number of misses will further decrease
Souresh’s score, and thus the only possible values are m=4m=4 and m=5m=5.

Solution 2

Since Souresh makes 20 throws and mm of these throws are misses, then the
remaining 20m20-m throws are
hits.

After exactly 20m20-m hits and mm misses, Souresh’s score is given by the
expression 7(20m)3m7(20-m)-3m or 14010m140-10m.

Since Souresh’s score (14010m140-10m) is
greater than 85, then 10m10m must be
less than 55 (note that $140 - 85 =
55)andso) and so m <
5510$.\frac{55}{10}\$.

Since 5510=512\frac{55}{10}=5\frac12 and
mm is a positive integer, then m5m\leq 5.

Since Souresh’s score is less than 105, then 10m10m must be greater than 35 (note that
140105=35140 - 105 = 35) and so m>3510m>\frac{35}{10}.

Since 3510=312\frac{35}{10}=3\frac12 and
mm is a positive integer, then m4m\geq 4.

Thus, mm is a positive integer and
4m54\leq m \leq 5, and so m=4m=4 or m=5m=5.

(We can check that when m=4m=4,
Souresh’s score is 7×163×4=11212=1007\times16-3\times4=112-12=100, and when
m=5m=5, Souresh’s score is 7×153×5=10515=907\times15-3\times5=105-15=90.)

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