The 15 equals the number of jellybeans Nico ate @ beginning. The 25 represents the "1/3" of the remaining jellybeans in the bag AFTER Nico ate 15, thus we can multiply 25 by 3 to find out how many jellybeans were left in the bag after Nico ate some and add our product to 15.
15 + (25*3) = j 15 + 75 = j 90 = j
So, there was a total of 90 jellybeans in the bag that Nico bought.