Bunuel wrote:
What is the number of integers from 1 to 1000, inclusive that are not divisible by 11 or by 35?
A. 884
B. 890
C. 892
D. 910
E. 945
The 30-second method: You can use a little number sense and PS technique to crack this one in no time at all. Ask yourself, how many times will 11 fit into
100? 9 times. This will repeat for each set of 100, since the upper limit is set at 1000.
\(9*10=90\)
So far, then, we have 90 numbers to remove. Now, repeat the process for 35, but with a little fine-tuning. How many times will 35 fit into
100? 2 times. Into 200? 5 times, since the second hundred starts at 105.
There cannot be more than three instances of 35 fitting into any given 100, since 3 * 35 = 105. For each set of
200, then, we should get about five 35s, and there are five sets of 200 in 1000. (We need not concern ourselves with the exact sequence of 2s and 3s per 200.)
\(5*5=25\)
There should be around 90 + 25, or 115 numbers to remove from consideration. Yes, there will be an overlap for each instance in which 11 and 35 cross paths, but even if 11 were 10, that would only happen twice out of our range of numbers.
\(1000-115=885\)
The answer
must lie within 2 of 885, so (A), 884, is the only option that works. We can choose (A) with 100 percent confidence and spare ourselves the mental energy we may need for the next challenge.
- Andrew
_________________
I am no longer contributing to GMAT Club. Please request an active Expert or a peer review if you have questions.