1
$\begingroup$

We have $400$ students in a school. Every $20^{th}$ student failed at the end of the school year. Which was $2\%$ of schools girls and $10\%$ of schools boys.

The number of all boys attending the school is?

What I tried:

We know that every $20^{th}$ student fails, which means that $20$ students have failed. We also know that in these $20$ students are $2\%$ of all girls from the school and $10\%$ of all boys from the school. So we can make two equations like this: $$400 = b + g$$ $$20 = 0.1b + 0.02g$$

--I know the answer is $150$, but I got a different one.. if someone could point out an error in my logic, or just show how to solve it?

$\endgroup$
4
$\begingroup$

Your equations are correct, so you apparently just made a mistake in solving the system.

I’d multiply the second equation by $10$ to get $200=b+0.2g$ and then subtract that from the first equation to get $200=0.8g$. Then $g=250$, so $b=400-250=150$.

$\endgroup$
  • $\begingroup$ Yeah, I made a stupid mistake, thanks :) $\endgroup$ – Mykybo Apr 10 '15 at 21:02
  • $\begingroup$ @Mykybo: It happens. You’re welcome! $\endgroup$ – Brian M. Scott Apr 10 '15 at 21:07
0
$\begingroup$

In your equation replace $g = 400 - b$ then you will get $b = 150$.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.