Explanation:

- In a class of 90 students, 60% are girls, which means there are 54 girls (0.6 x 90).

- The remaining students are boys, so there are 36 boys (90 - 54).

- Average marks for boys are 63.

- Average marks for girls are 70.

- To find the average marks of the whole class, calculate the total marks as follows:

- Total marks for girls: 54 x 70 = 3780

- Total marks for boys: 36 x 63 = 2268

- Combine the two to get total marks for the class: 3780 + 2268 = 6048

- Average marks for the class: 6048 / 90 = 67.2

Option 3, 67.2 is the correct answer.