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Byju's Answer
Standard X
Mathematics
Formula for Sum of n Terms of an AP
The sum of al...
Question
The sum of all 2-digit numbers divisible by 5 is
(a) 1035
(b) 1245
(c) 1230
(d) 945
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Solution
(d) 945
The 2-digit numbers divisible by 5 are 10, 15, 20, 25, ..., 95.
This is an AP in which a = 10, d = (15 - 10) = 5.
Let the number of terms be n.
Then T
n
= 95
⇒ a + (n - 1)d = 95
⇒ 10 + (n - 1) ⨯ 5 = 95
⇒ 5n = 90
⇒ n = 18
∴ S
18
=
n
2
a
+
l
=
18
2
×
10
+
95
=
9
×
105
=
945
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