Question 549428
Assign variables based on their names.
We know that:
d+m+j=39
We also know that 
d=1/2m
and 
d=j-3

We can find from that that 
m=2d
and 
j=d+3
Lets sub those in
d+2d+d+3=39
4d+3=39
4d=36
d=9
therefor j=12 and m=18