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Billyneutron
Bunuel
Official Solution:

Lisa wrote down 10 consecutive integers on a blackboard. However, Bart, being mischievous, erased one of them. If the sum of the remaining nine numbers is 146, what is the value of the number that Bart erased?

A. 7
B. 9
C. 12
D. 19
E. 21


Assume the smallest of the 10 consecutive integers is \(x\). Then their sum is:

\(x + (x + 1) + ... + (x + 9) = 10x + (1 + 2 + ... 9) = 10x + \frac{1 + 9}{2}*9 = 10x + 45\).
Now, let's assume Bart erased the number \(k\). Then, we'd have:

\(10x + 45 = 146 + k\);

\(10x = 101 + k\).
Since \(x\) is an integer, the left-hand side of the equation is a multiple of 10. Therefore, the right-hand side must also be a multiple of 10, which means the units digit of \(k\) must be 9. This narrows our options down to B (9) and D (19). However, \(k\) cannot be 9 because in that case \(x\) would be 11, and the erased number cannot be smaller than the smallest number in the set. Therefore, Bart must have erased the number 19.


Answer: D
­
Hello, I could not understand the part where 9 is eliminated; pls explain, thanks
­If k = 9, then x = 11, so the consecutive integers are 11, 12, ..., 20. As you can see, this sequence does not contain 9.­
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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With that calculation we got x = 13; and last element x+9 = 22;

Sum of all the elements in the series = (13+22)*10/2 = 175

Now, if 19 got removed from the 175 we got = 175-19 = 156, not 146?
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With that calculation we got x = 13; and last element x+9 = 22;

Sum of all the elements in the series = (13+22)*10/2 = 175

Now, if 19 got removed from the 175 we got = 175-19 = 156, not 146?
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x = 12, not 13.
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What a brainstormer!!!
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