find the sum of all three digits number which leave the remainder 2 when divided by 5. 

Three digit numbers which leave the remainder 2 when divided by 5 are 102, 107, 112,......., 997.

102, 107, 112,......, 997 is an A.P

First term of the A.P, a = 102

Common different of the A.P, d = 5

Let 997 be the n^th term of the A.P.

a_n = a + (n – 1) d

∴ 102 + (n – 1) × 5 = 997

⇒ 5 (n – 1) = 997 – 102 = 895

⇒ (n – 1) = 179

⇒  n = 180

Sum of all three digit numbers which leaves remainder 2 when divided by 5