The cross box is the number A in the middle of five numbers on the calendar.

These five numbers are: a- 1, a, a+ 1, a-7, a+7,

The sum of five numbers is x: a-1+a+a+1+a-7+a+7 = 5a = x,

X is a multiple of 5.

∫a+7≤3 1，

∴a≤24，

∴x=5a≤ 120.

So choose a.