Question 696511: Find the sum of all 4 digit natural numbers of the form 4AB8 which are divisible by 2,3,4,6,8, and 9
Answer by Edwin McCravy(20054) (Show Source):
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To be divisible by 2,3,4,6,8, and 9
we list all the like factors in
columns to get the least common
multiple of all those:
2= 2
3= 3
4= 2·2
6= 2 ·3
8= 2·2·2
9= 3·3
----------------
2·2·2·3·3 = 72
So we find all the multiples of 72
that are 4-digits, that begin with a
4 and end with an 8.
The smallest feasible number is
4008, which divided by 72 gives 55.66,
a decimal, not a whole number.
The largest feasible number is
4998, which divided by 72 gives 69.42,
also a decimal, not a whole number.
Of course neither of those qualify,
but they tell us the bounds to look
for numbers to multiply by 72. Since
72 ends in 2, to get a number
ending in 8, we know that a number
ending in 4 will give an answer
ending in 8, since 2·4 = 8. Also we
know that a number ending in 9
multiplied by 72 will give an answer
ending in 8 since 2·9 = 18.
The only two digit numbers in that
range which end in 4 or 9 are
59, 64, and 69. Multiplying them by
72 gives the answers
72·59 = 4248
72·64 = 4608
72·69 = 4968
Add them up and get 13824
Edwin
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