Find the sum of two digit numbers that are completely divisible by 3
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≡QUESTION≡
Find the sum of two digit numbers that are completely divisible by 3.
║⊕ANSWER⊕║
The 2 digits no: include
12, 15, 18, ..... 99
a = 12
d = 3
aₙ = 99
n = ?
a + (n-1)d = aₙ
12 + (n-1)3 = 99
(n-1)3 = 99 - 12
n-1 = 87/3
n = 29+1
n = 30
Sₙ = n/2 (a + aₙ)
=30/2 (12 + 99)
= 15 (111)
= 1665
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