Answer :
I am pretty sure the answer is 15 boys at the beginning of the assembly.
That would mean that there were 28 girls there, and if 40% of them left, which would mean that 8 left, leaving 20 girls. This would leave the assembly with a 3:4 ratio of boys to girls.
That would mean that there were 28 girls there, and if 40% of them left, which would mean that 8 left, leaving 20 girls. This would leave the assembly with a 3:4 ratio of boys to girls.
There were 15 boys at the beginning of the assembly and 28 girls