Answer :
The n-th term of an arithmetic sequence with first term a and common difference d is
a + (n - 1) d
So, the 51st term of A is
30 + 9 (51 - 1) = 480
while the 51st term of B is
30 + 4 (51 - 1) = 230
The difference between them 480 - 230 = 250.