Let's use algebra to solve this problem. Let "b" represent the number of boys and "g" represent the number of girls.

According to the given information, there are 100 boys and girls in total, so we can write the equation:

b + g = 100

Each boy receives Tk 3.6, and since there are "b" boys, the total amount they receive is 3.6b. Similarly, each girl receives Tk 2.4, and since there are "g" girls, the total amount they receive is 2.4g. The total sum of money is Tk. 312, so we can write another equation:

3.6b + 2.4g = 312

Now, we have a system of two equations:

1. b + g = 100
2. 3.6b + 2.4g = 312

We can solve this system of equations simultaneously. Let's start by multiplying the first equation by 2.4 to match the coefficients of "g" in both equations:

1. 2.4b + 2.4g = 240

Now, we can subtract equation 1 from equation 2:

(3.6b + 2.4g) - (2.4b + 2.4g) = 312 - 240

This simplifies to:

1.2b = 72

Now, divide both sides by 1.2 to solve for "b":

b = 72 / 1.2 b = 60

Now that we know the number of boys (b), we can find the number of girls (g) using the first equation:

b + g = 100 60 + g = 100

Subtract 60 from both sides:

g = 100 - 60 g = 40

So, there are 40 girls.