rohan2345 wrote:
The average of six consecutive integers in increasing order of size is \(9 \frac{1}{2}\) . What is the average of the last three integers?
(A) 8
(B) 9 1/2
(C) 10
(D) 11
(E) 19
I like this question because it forced me to choose between a decent bit of algebra and logic/common sense.
List the numbers. We know 9.5 is the average, and because we have consecutive integers where mean = median, we know 9.5 is the "middle term."*
So thinking about a number line, 9.5 will be between the
consecutive integers 9 and 10.
From there, you've used two of the six integers. They're consecutive. You need to use up the other four integers, and they need to "balance" so that 9.5 stays in the middle.
It's then easy to see that the numbers, which must be integers, are 7, 8, 9, 10, 11, 12
You can find the average of the last three numbers either by remembering that 10, 11, 12 is an arithmetic sequence, too, so median = mean (average), hence 11, OR
\(\frac{10 + 11 + 12}{3}\) = \(\frac{33}{3}\)= 11
*Also, with an even number of terms, the average will be the average of the two middle terms, so there will be three
consecutive integer terms to the right of 9.5, and three to the left of 9.5. Even if this fact doesn't pop into your brain, once you start listing the first two numbers, that fact will become apparent.
Answer D. Hope it helps.