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