3/4 of 1800 is [tex]\displaystyle{ \frac{3}{4}\cdot1,800 = 3\cdot450=1,350[/tex].
1/5 of 1800 is [tex]\displaystyle{ \frac{1}{5}\cdot1,800 = \frac{1,800}{5}=360[/tex]
Thus, out of 1,800 members, 1,350 are aged 25-59, 360 are aged over 60.
The remaining, 1,800-1,350-360 = 90, are members under 25 years old.
Answer: 90
