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Explanation:
The process of finding the arithmetic mean involves two steps
The first bit of info your teacher gave you is "the mean of five numbers in a set is 12". While we don't know the actual individual values themselves, we know that they must add to some variable y. Dividing y over n gets us the mean
y/n = mean
y/5 = 12
y = 5*12
y = 60
The five values add to 60.
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The next bit of info is that "the mean of all six is 14". That allows us to say:
(sum of all 6 values)/6 = mean
(sum of all 6 values)/6 = 14
( (sum of 5 values) + (6th value) )/6 = 14
(y + x)/6 = 14
(60 + x)/6 = 14
where x is the 6th number we want to find.
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Solve for x
(60 + x)/6 = 14
60+x = 6*14
60+x = 84
x = 84-60
x = 24 so choice (1) is the answer