The ratio of the number of boys and girls in a school 432 students is 5 : 4. How many new girls should join the school so that the ratio becomes 1 : 1?
The Usual Method
The number of boys in the school =
The number of girls in the school =
For the ratio of boys and girls to become 1 : 1, the number of new girls that join the school should be = 240 – 192 = 48
Estimated Time to arrive at the answer = 45 seconds.
To have equal number of boys and girls, the ratio of boys to girls should change from 5 : 4 to 1 : 1. Also, 1 : 1 is equivalent to 5 : 5. The number of boys = 5x and the number of girls = 5x. Hence total number of students = 10x.
This means that the total number of students after new girls join is a multiple of 10 and hence the last digit of the sum should be = 0.
The existing total number of students = 432 with last digit = 2. Hence, the last digit of the number of girls who join should have last digit = 8, to get the last digit of the final sum = 0. The only option satisfying this condition is option ‘2’.
Estimated Time to arrive at the answer = 10 seconds.