Solution :
First term = 125
a = 125
Common difference = t2 – t1
= 120 – 125
= -5
General term of an A.P
(tn) = a + (n - 1) d
15th term of the A.P = 125 + (15 - 1) (-5)
= 125 + 14 (-5)
= 125 – 70
t₁₅ = 55
Hence, 15th term of A.P is 55