Question 1123041
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The quick answer given by tutor @ikleyn uses a shortcut that is often useful in problems involving arithmetic sequences.  (An arithmetic  sequence is a sequence of numbers in which there is a constant difference from one number to the next.  A sequence of consecutive integers is a common example.)<br>
Because of the equal spacing between the numbers in an arithmetic sequence, the middle number is the average of all the numbers.  In a sequence of 5 numbers, the 3rd number is the middle number.<br>
So in your problem the third number is the average of the 5 numbers -- which is the sum of the numbers, divided by how many there are: 505/5 = 101.