there are 42 boys present and 1 boy absent (43 boys in all)
I subtract the number of absentee/s ===> 64-1=63
then divide it to 3 cause "twice more boys than girls". ==> 63 ÷ 3 = 21
Multiply 21 to 2 ====> 21× 2 = 42
21+42=63 (plus one for the absent cause it says "64 kids enrolled in a class."
so ....
42 are boys, while there are 21 girls.