When you reverse the digits of a 2-digit number, the value changes by 9 times the difference in the values of the digits. The problem statement is telling you the difference of the two digits is 27/9 = 3 and the sum of the two digits is 5. Then the digits must be 4 and 1.
John's father is 41.
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John is 14.
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This is two problems in one, a "reversed digits" problem and a "sum and difference" problem.
If you have digits x and y, when x is the 10s digit, the value is (10x +y). When y is the 10s digit, the value is (10y +x). The difference of these is (10x +y) -(10y +x) = 9x -9y = 9(x -y).
The other problem is a "sum and differenc" problem. If the sum of two numbers (a, b) is s and their difference is d, the numbers can be found from ...
.. a +b = s
.. a -b = d
add these equations:
.. 2a = (s +d)
.. a = (s +d)/2 . . . . . . the larger number is the average of the sum and the difference
.. b = a -d = s -a = (s -d)/2 . . . . . you can find the other number several ways
