Mock T1/P2 Q7

There are 315 pupils in a school.
3/7 of the boys and 3/4 of the girls do not wear watches.
The number of boys who wear watches is twice the number of girls who wear watches. How many girls are there altogether?

Model method:

Those who wear watches - 4/7 of the boys and 1/4 of the girls.
4/7of the boys = 1/4 of the girls

Using model, draw 2 units for boys and 1 for girls.

For girls, add 3 more units (total of 1/4+3/4)
For boys add 1.5 units (total of 4/7 and 3/7)
Altogether 15 units = 315
1 unit =21
Girls = 8 units = 168


Units method:

Boys==> Wear: No wear = 4:3 ==> 4:3
Girls==> Wear: No wear = 1:3 ==> 2:6
Since the boys who wear = twice the girls who wear, make the ratio equivalent (girls = 1/2 units of 4=2)

Total units = 4+2+3+6=15
15 units = 315
1 unit = 21
Girls total 8 units (2+6) = 8x21 = 168