The boy ans girls in a college are in the ratio 3: 2. If 20% of the

I. Logic - 15 seconds
Boys, % NOT adult: (100 - 20) = 80%
Girls, % NOT adult: (100 - 25) = 75%

Percent of all students who are not adults? MUST be between 75% and 80%. One answer works

Answer C

II. Weighted averages

Method #1
(B:G) = (3:2), total ratio parts 5
Not adults = N.A.
B's "weight" = 3
G's "weight" = 2

(% N.A.)(Weight of B) + (% N.A.)(Weight of G) divided by ratio parts' total of 5

\(\frac{.8(3)+.75(2)}{5}=\frac{2.4+1.5}{5}=\frac{3.9}{5}=\)
\(\frac{39}{50}=\frac{78}{100}=78\) %

Method #2
Finally, you can weight the boys and girls by their percent of the population

(B:G) = (3:2), total of 5 parts
B = \(\frac{3}{5}\) = 60% = (.6 of all students)
G = \(\frac{2}{5}\) = 40% = (.4 of all students)

From above, take the percent of boys and girls who are NOT adults. Multiply B and G by their respective percent of the whole population. Add.

No need to divide. That's accounted for when you take the percent of the population.

\((.6)(.8) + (.4)(.75) = (.48 + .30) = 78\) %

ANSWER C