let x= the one you don' t have any information therefore
x= 3 yr olds
1/2 x= 2 yr olds
x= 4 yr. old
x+1/2x+x=45
2x+1/2x=45
make the fraction a whole number by multiplying the equation by 2
4x+x=90
5x=90
x=18 # of 3yr olds
1/2(18)=9 # of 2 yr olds
x=18. # of 4 yr olds