45% of the pupils in a school are Chinese. 35% of them are Malays and the remaining 420 pupils are are Indians. What is the total enrolment of the school?
If N = total number, then 420 represents (100 - 35 - 45)% (=20%) of the total, so 20%N = 420 or 0.2N = 420
N = 420/0.2 or N = 2100
.