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?

explain how to get this answer.

Boys to girls ratio was 3:4 which means out of every 3+4= 7 students, 3/7 are boys and 4/7 are girls. But 40% of the girls left so only 60% are the 4/7. 60% is 0.6 in decimal form so to find how many women are really in the ratio (4/7)/0.6=0.95
the new ratio is 3/7:0.95 or 1:2.22 simplified so the number of boys b is b=w/2.22