Answer:
a = 12.
Explanation:
Sum of n terms = n/2[2a + d(n-1)]
For 25 terms
S25 = 12.5(2a + 24d)
S25 = 25a + 300d
For 4 terms
S4 = 2(2a + 3d)
So:
S25 = 15*2(2a + 3d)
S25 = 60a + 90d
25a + 300d = 60a + 90d
35a = 210d
a = 6d
Take a to be 12 and d to be 2:
25th term = 12.5(2*12 + 24 * 2) = 900
4th term = 2(24 + 6) = 60
900 = 15 * 60 so a = 12.