You can work out the pattern by subtracting the first number from the second. So for the first one, 7 1/2 (Which is equal to 7 2/4, because 1/2 = 2/4) - 7 1/4 = 1/4, so you add a quarter each time
For the second one, 8 - 8 1/3 = -(1/3), so you subtract a third each time
Hope that helps and isn't too confusing!