Question

There are 1800 gym members in total, 3/4 are aged 25-59, 1/5 are over 60 and the remainder are under 25, how many members are under 25years

Answer 1

If $x$ is the number of members under 25, then we have

$1800=\dfrac34\cdot1800+\dfrac15\cdot1800+x\implies x=90$