Rudy is training for a race. on monday he ran 2
km. on tuesday he ran 15% farther than on monday.
if he increases his running distance by the
same percentage each day, on which day will he
first run more than 4 km?

Distance ran by Rudy on Monday = 2 km
Percentage of increase in distance of running each day = 15%
Let us assume that the day on which Rudy will run 4 km = x
Then
2 + 0.15x = 4
0.15x = 2
x = 2/0.15
   = 13.3 
From the above deduction we can conclude that Rudy will run more than 4 km on the 14th day or on Sunday. I hope the procedure is clear enough for you to understand. 

X=3 and y=4 is the answer