Explanation:

- The ratio of boys to girls is given as 35:39. This means for every 35 boys, there are 39 girls.

- The combined average number of boys and girls is 740. This average is calculated by adding both groups and dividing by 2.

- To find the total number of girls, you start by adding the ratio numbers: 35 + 39 = 74.

- Represent the boys as 35x and the girls as 39x since their total relation in ratio terms is 74x.

- The average means 74x divided by 2 equals 740. So, 74x = 1480. Solving for x gives x = 20.

- Multiply x by the girls' ratio: 39 * 20 = 780.

The number of girls in the school is 780.