There are four known numbers, 7, 7, -3, 7.

Then 7+7-(-3)+7 equals 24.

Decomposition:

7+7= 14;

14-(-3)= 17;

17+7=24。

If the number provided by the author is a keyboard input error and the dot "."is a comma ",",that is, the original question is, 7.7, -3.7, how to make it equal to 24?

Addition and subtraction are as follows:

Analysis: Known 7.7, -3.7, equal result 24.

Suppose there is an unknown x in the middle,

Then 7.7-3.7+X=24

4+X=24

X=20