When you think least common denominator, what you should really be thinking is least common multiple of the denominators.
In this case, our denominators (bottom numbers) are 25 and 10.
Let's find the LCM.
multiples of 10: 10, 20, 30, 40, 50...
multiples of 25: 25, 50, 75, 100...
The LCM/LCD is 50.