You have that on planet Alaber, there are 15 dubbles to every 13 rews. This proportion can be wrtten as 15:13, or 15/13.
In order to calculate how many dubbles are on the farm, while there are 100 rews. You use the previous ratio and proceed as follow:
15/13 = x/100 where x is the unknown number of dubbles
This is because the ratio between dubbles and rews must be the same.
You solve the previous equation as follow:
15/13=x/100 multiply both sides by 100 to cancel the denomitaro 100 right side
15/13(100) = x/100(100)
1500/13 = x
In order to write the previous result as a mixed number you divide numerator and denominator:
1500 | 13
143 115
70
65
5
Then, x = 1500/13 is also equal to:
x = 115 13/5
This means there are approximately 115 dubbles for 100 rews