Answer:
13.5, 17.5
Explanation:
You have two numbers whose sum is 31 and whose difference is 4. The second number is larger.
Setup
Let x represent the larger number. Then the smaller is x-4, and their sum is ...
x +(x -4) = 31
Solution
Simplifying the expression gives ...
2x -4 = 31
2x = 35 . . . . . add 4
x = 17.5 . . . . . divide by 2
x -4 = 13.5
The two numbers are 13.5 and 17.5.
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Additional comment
As we have seen here, the larger number is half the total of the 'sum' and 'difference'. (31+4)/2 = 17.5. This is the generic solution to any "sum and difference" problem.