Maths Olympiad Prep

Track / Stage 6 / 370 of 400 #1370 of 1964

Problem 1370

National olympiad, first round
Algebra Difficulty 6.8 Find the answer

In one class in the school, number of abscent students is 16\frac{1}{6} of number of students who were present. When teacher sent one student to bring chalk, number of abscent students was 15\frac{1}{5} of number of students who were present. How many students are in that class?

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

1. Let x x be the number of students who were present initially.
2. Let y y be the number of students who were absent initially.

From the problem, we know:
y=16x y = \frac{1}{6}x

3. When the teacher sent one student to bring chalk, the number of present students becomes x1 x - 1 , and the number of absent students remains y y .

4. At this point, the number of absent students is 15 \frac{1}{5} of the number of present students:
y=15(x1) y = \frac{1}{5}(x - 1)

5. We now have two equations:
y=16x y = \frac{1}{6}x
y=15(x1) y = \frac{1}{5}(x - 1)

6. Substitute y y from the first equation into the second equation:
16x=15(x1) \frac{1}{6}x = \frac{1}{5}(x - 1)

7. To eliminate the fractions, multiply both sides by 30 (the least common multiple of 6 and 5):
3016x=3015(x1) 30 \cdot \frac{1}{6}x = 30 \cdot \frac{1}{5}(x - 1)
5x=6(x1) 5x = 6(x - 1)

8. Simplify and solve for x x :
5x=6x6 5x = 6x - 6
5x6x=6 5x - 6x = -6
x=6 -x = -6
x=6 x = 6

9. Substitute x=6 x = 6 back into the first equation to find y y :
y=166 y = \frac{1}{6} \cdot 6
y=1 y = 1

10. The total number of students in the class is the sum of the present and absent students:
x+y=6+1=7 x + y = 6 + 1 = 7

The final answer is 7\boxed{7}

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