Explanation:

- The ratio of boys to girls is given as 35:39.

- Let the number of boys be 35x and the number of girls be 39x.

- The average of boys and girls in the school is given as 740.

- This means: $\frac{35x+39x}{2}=740$.

- Simplifying the equation gives: $\frac{74x}{2}=740$.

- Further simplification: 37x = 740.

- Solve for x: x = 20.

- The number of boys is 35x = 35 * 20 = 700.

- Therefore, the correct option is 3: 700.