Maths Olympiad Prep

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Algebra Difficulty 3.6 AMC 10/12 Find the answer

Laila took five math tests, each worth a maximum of 100 points. Laila's score on each test was an integer between 0 and 100, inclusive. Laila received the same score on the first four tests, and she received a higher score on the last test. Her average score on the five tests was 82. How many values are possible for Laila's score on the last test?

Pick one

Solutions — 2

Solution 1

Say Laila gets a value of xx on her first 4 tests, and a value of yy on her last test. Thus, 4x+y=410.4x+y=410.
The value yy has to be greater than 8282, because otherwise she would receive the same score on her last test. Additionally, the greatest value for yy is 9898 (as y=100y=100 would make xx as a decimal), so therefore, the greatest value xx can be is 9898. As a result, only 44 numbers work, 86,90,9486, 90, 94 and 9898. Thus, the answer is (A) 4\boxed{\textbf{(A) }4}.

Solution 2

1. Let x x be Laila's score on each of the first four tests, and let y y be her score on the last test. Given that her average score on the five tests was 82, we can set up the following equation:
4x+y5=82 \frac{4x + y}{5} = 82
2. Multiply both sides of the equation by 5 to clear the fraction:
4x+y=410 4x + y = 410
3. We need to find integer solutions for x x and y y such that 0x100 0 \leq x \leq 100 and 0y100 0 \leq y \leq 100 , with the additional condition that y>x y > x .

4. Rearrange the equation to solve for y y :
y=4104x y = 410 - 4x
5. Since y y must be greater than x x , we have:
4104x>x 410 - 4x > x
6. Combine like terms:
410>5x 410 > 5x
7. Divide both sides by 5:
x<82 x < 82
8. Since x x must be an integer, the possible values for x x are 0,1,2,,81 0, 1, 2, \ldots, 81 .

9. For each value of x x , we calculate y y using y=4104x y = 410 - 4x . We need y y to be an integer between 0 and 100, inclusive.

10. To find the range of y y , we substitute the minimum and maximum values of x x into the equation y=4104x y = 410 - 4x :
- When x=0 x = 0 :
y=4104(0)=410 y = 410 - 4(0) = 410
This value is not within the range 0 to 100.
- When x=81 x = 81 :
y=4104(81)=410324=86 y = 410 - 4(81) = 410 - 324 = 86
This value is within the range 0 to 100.

11. We need to find the values of y y that are greater than x x and within the range 0 to 100. Since y=4104x y = 410 - 4x , we need:
86y100 86 \leq y \leq 100
12. To find the corresponding values of x x for y y in this range, we solve:
- For y=86 y = 86 :
86=4104x    4x=324    x=81 86 = 410 - 4x \implies 4x = 324 \implies x = 81
- For y=90 y = 90 :
90=4104x    4x=320    x=80 90 = 410 - 4x \implies 4x = 320 \implies x = 80
- For y=94 y = 94 :
94=4104x    4x=316    x=79 94 = 410 - 4x \implies 4x = 316 \implies x = 79
- For y=98 y = 98 :
98=4104x    4x=312    x=78 98 = 410 - 4x \implies 4x = 312 \implies x = 78

13. Therefore, the possible values for y y are 86,90,94,98 86, 90, 94, 98 .

The final answer is 4\boxed{4}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.