3/10 of them are adults

So fraction of people that are boys and girls

= 1 - 3/10
= 7/10

Since there were equal number of boys and girls at first,

boys make up ½ of their numbers, girls make up the other ½.

So fraction of people that are boys

= 7/10 x ½

= 7/20

= fraction of people that are girls

Notice that no girls left the pool. So the number of girls remained the same.

This is a '1 item unchanged' type of question.


Now 80% of the remaining people were girls.

And 80% = 80/100 = 4/5


So 4/5 of remaining people = 7/20 of people at first.

Now make the numerator the same since the units must be the same
(Equal concept)

Common multiple of 4 and 7 is 28
(4 x 7 = 28, 7 x 4 = 28)


Multiply by 7,
4/5 = 28/35

Multiply by 4,
7/20 = 28/80


So 28/35 of remaining people = 28/80 of people at first.


From the denominators, we can tell that there are 35 units of people remaining and 80 units of people at first.


Difference = 80 units - 35 units = 45 units

Difference also = number of people who left = 90

So 90 people = 45 units

1 unit = 90 people ÷ 45 = 2 people

Number of girls = 28 units

= 28 x 2
= 56

Thank you for the detailed explanation and working. They are helpful to my understanding of the concept of one item unchanged.

Welcome