The price of a shirt is 5/6 of the price of a pair of pants. the shirt costs $12.50. how much do the pants cost?

Let y be the cost of the pants. 
We can use algebras to solve this, 
from the given information, we can write the following equation:

y x 5/6 = 12.50

To calculate the amount of an item if the item is (fraction) of another item, just multiply the fraction with the latter. 

Now time to solve this. 

Shift x( 5/6) to the other side, turning to ÷(5/6)

y = 12.5 ÷ 5/6
y = 15

Therefore, the cost of the pants are $15