Question 364315
Here is what I came up with...

Step 1: Let G equal the number of hours Gabriel stayed at the party

Step 2: G + G - .50 + G + 1.50 = 4

Step 3: G = 1

Step 4: 1 + 1 - .50 + 1 + 1.50 = 4

Step 5: Gabriel stayed at the party for 1 hour, Collin stayed at the party for a half an hour, and Terrance stayed at the party for 2 and a half hours. 

How I came up with that formula:

Gabriel + Gabriel - .50 hours (this is Collin) + Gabriel + 1.50 (this is Terrance) = 4 (Total hours)