SOLUTION: Calculate the mean A and the median B of this set of fifteen automobile speeds collected by a police officer: 72; 80; 62; 71; 71; 64; 65; 73; 66; 78; 81; 66; 64; 77;

To find the mean

1. Add them
2. Divide by the number of numbers

1.  72+80+62+71+71+64+65+73+66+78+81+66+64+77+66 = 1056
2.  1056÷15 = 70.4 

That's the mean, 70.4

To find the median of an odd number of numbers.

1.  Arrange them in order smallest to largest
2.  Pick out the number that's right in the middle.

1.  62; 64; 64; 65; 66; 66; 66; 71; 71; 72; 73; 77; 78; 80; 81
2.  The number that's right in the middle of the list is the
    one I've colored red.  It's 71.

A - B = average - median = 70.4 - 71 = -0.6  which is choice B

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Incidentally when there are an even number of numbers, to
find the median, there is no one number right in the middle,
as there was here.  Instead there are TWO numbers right in 
the middle, and you have to add them and divide by 2 in 
order to get the median.  So it's not quite as easy to find
the median when there are an even number of numbers as it
is when there are an odd number.

Edwin