The mean on a normal curve will be exactly in the middle, so on this curve the mean, or m, is 56. One standard deviation, or d, is 13 trips, so if we move to the left to m  d, we will be at 43 trips, the upper boundary of what the question is asking about. Moving a further 13 trips to the left will get us to m  2d, which is thus 30. Since they're asking how many took between 30 and 43 trips, we know 14% of the population did so.
So what is 14% of 800,000? There are a couple ways to quickly estimate this. One is to know that 14% is about the same as 1/7th, and 1/7th of 800,000 should be a little over 100,000, since 1/7th of 700,000 would be exactly 100,000. Answer choice B looks pretty close.
Another way to estimate would be to say that 14% of a million would be 140,000, but our answer choice should be a little less than that. Again answer choice B is the only choice anywhere near our estimate.
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