it will be 50, in the question it is mention that the "the attribute value is uniformly distributed",
there are hundred tuples and there are 20 elements in set 100/20 = 5, i.e., for each attribute value there are 5 rows.
not less than 10 means 9*5 = 45, but it is given less than and equal to, 10*5 = 50. BTW which is equal for (>10) too.