When 40% of the girls left the school assembly, the ratio of the number of boys to girls became 3 to 4. If there were w girls when the assembly started, what was the total number of boys when the assembly started?

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.

There were 15 boys at the beginning of the assembly and 28 girls