A school has 1650 students, and the number of boys is 20% more than the number of girls. The number of girls is:

Let there be g girls; then the boys number 1.2g and g + 1.2g = 2.2g = 1650 → g = 750 (and 900 boys). Check: 900 is indeed 20% more than 750. (Taking 20% off the half-and-half figure of 825, or splitting 1650 in the ratio 60 : 40, both use the 20% on the wrong quantity — it is measured on the girls, not on the total.)