Explanation

Let's break down the problem step by step:

Step 1: Express the initial number of boys and girls using the given ratio

Let the number of boys be 11x and the number of girls be 13x.

Step 2: Calculate the new number of boys and girls after 4 boys and 8 girls join the class

New number of boys = 11x + 4

New number of girls = 13x + 8

Step 3: Set up the equation based on the new ratio

Given the new ratio is 4:5, we can write the equation as:

(11x + 4) / (13x + 8) = 4/5

Step 4: Solve the equation

Cross-multiplying gives us:

5(11x + 4) = 4(13x + 8)

55x + 20 = 52x + 32

55x - 52x = 32 - 20

3x = 12

x = 4

Step 5: Calculate the strength of the class

Strength of the class = Initial number of boys + Initial number of girls + New boys + New girls

= 11x + 13x + 4 + 8

= 24x + 12

= 24*4 + 12

= 96 + 12

= 108