at first club meeting 60% of the students were boys, if there were one fewer girl than boy in attendance, how many students attended the meeting

I assume u mean how many boys had attende the meeting. there are 60% boys, so lets think that there are 10 students where 6 are boys and four are girls.

if there had been one fewer girl we would have 9 students where there will be 6 boys.

boys: 6/9 = > 2/3 66.66% of the students are boys.