Question 1143708
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The solution by tutor @ikleyn shows what is probably the standard way for solving a problem like this.<br>
Here is another method that I often use in problems like this involving averages of numbers that are "close together".<br>
Try it and see if it "works" for you.<br>
(1) The new number is 4.2-3.5 = 0.7 greater than the current average.
(2) To get the new average, that additional 0.7 has to be divided evenly among what is now 7 numbers.
(3) That means the new average is larger by 0.7/7 = 0.1 -- so the new average is 3.5+0.1 = 3.6.