Question

find the sum of all two digit positive integers which are divisible by 3 but not by 2​

Answer 1

All two digit positive integer which is divisible by 3 but not 2 are 15,21,27 ... 99
Here a = 15 d = 21 - 15 =6 l = 99
Tn = a + (n- 1)d
99 = 15 + (n - 1)6
99 = 15 + 6n - 6
90 = 6n
n = 15
Now Sn = n /2 (a + l)
S15 = 15 / 2 ( 15 + 99)
S15 = 15/2 (114)
S15 = 855
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