Let's first find the number of Christian students and Hindu students in the school.

Let the total number of students be "x."

Percentage of Hindus = 75%
Percentage of Christians = 100% - 75% = 25%

Number of Hindu students = 75% of x = 0.75x
Number of Christian students = 25% of x = 0.25x

Next, let's find the number of girls and boys among Hindus and Christians.

Percentage of girls among Hindus = 16%
Percentage of boys among Hindus = 100% - 16% = 84%
Percentage of girls among Christians = 40%
Percentage of boys among Christians = 100% - 40% = 60%

Number of girls among Hindus = 16% of Hindu students = 0.16 * 0.75x = 0.12x
Number of boys among Hindus = 84% of Hindu students = 0.84 * 0.75x = 0.63x
Number of girls among Christians = 40% of Christian students = 0.40 * 0.25x = 0.10x
Number of boys among Christians = 60% of Christian students = 0.60 * 0.25x = 0.15x

We know that the total number of boys in the school is 390. So, the sum of boys among Hindus and Christians is 390.

Total number of boys:
0.63x + 0.15x = 390
0.78x = 390
x = 390 / 0.78
x = 500

Now, we have found the total number of students (x = 500).

Finally, the total strength of the school is 500 students.