The ratio of boys to girls at a college is 4:5. How many boys and are there if the total num of students is girls 3321?

There is an important difference between the way counts are viewed in ratio to that in fractions.

The ratio of boys : girls #-> 4:5# so you have 4 boys and 5 girls making the whole count #4+5 = 9# in its simplest form.

So to change the ratio proportion into fraction proportion we have:

boys #-> 4/9# of the whole.

girls #->5/9# of the whole.

It is given that the whole is 3321 therefore the count of boys is:

#4/9xx3321 = 1476#

There is a problem with interpreting the question part:
"the total num of students is girls 3321"

It would make more sense if worded:

#color(brown)("Case 1. 'if the total num of students is 3321' ")#

#color(brown)("Case 2. 'if the total num of girls is 3321'")#

My other answer assumes case 1
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#color(blue)("Answering as if Case 2 is applicable")#

Using ratio in fraction FORMAT.

#color(brown)("Note that this is not fractions of the whole.")#

#("boys")/("girls") -> 4/5-=x/3321#

Multiply both sides by 3321

#x=2656.8#

This #ul("answer does not make sense")#

You can not have part of a boy. So the other solution where I address Case 1 must be the correct interpreatation