The problem doesn't say that any girls left, so we'll assume that no girls
left, and so there were also g girls at the beginning of the program.

Let x = the number of boys at the beginning of the program.

Then we subtract one-fifth of x from x to get b boys: 



Multiply through by 5:







Checking:
There were  boys at the beginning, then  left,
and so there were  boys at the end.

Answer:  boys and g girls at the beginning.

Edwin