This is a pretty simple question. We know for sure the number is most probably under 30. So you can guess in check. Such as this:
Number: 28
1/4 of 28 = 7
21 - 1/2 = 10.5
So it doesnt fit the equation, so it isn't number 29
Number: 18
1/4 of 18 = 4.5
1/2 of 14.5 = 7.25
So it doesnt fit the equation, so it isn't number 18. Since 7.25 is close to six it's probably around the range 12-17
Number: 12
1/4 of 12 = 3
1/2 of 9 = 4.5
So it doesnt fit the equation, so it isn't number 12. Since the number was under, and 18 was above, it probably is 14-17
Number: 14
1/4 of 14: 3.5
1/2 of 11.5: 5.75
So it doesnt fit the equation. So it isn't number 14. Since the number was under, and 18 was above, it is probably 17
Number: 17
1/4 of 17: 4.25
1/2 of 13.75
This is above, but if you can round, as this is a decimal, this can count. Now though you should try decimals like this:
16.1,16.2,16.3,16.4, but if you can round the answer is the number 17 READ MY EXPLANATION IK YOU PROBABLY WONT