The answer is 18.
Solution:
30*3=90
90/5=18
Another solution:
30/5=6
6*3=18
The sum of the numbers should be divisible by 8 since 5+3=8. It means that the sum of the two numbers can be divided into 8 parts. The first ratio is 3/8 while the second is 5/8.
Checking:
18+30=48
48/8=6
3*6:5*6
18:30
