155226

The hollow squares can be divided up into 4 rectangles, thus:
960/4=240

To find the different combinations of rectangle sides, we could use factor pairs:
1*240
2*120
3*80
4*60
5*48
6*40
8*30
10*24
12*20
15*16

1) outer square - 240+1 = 241
inner square - 240-1=239

2) outer square - 120+2 = 122
inner square - 120-2 = 118

3) outer square - 80+3 = 83
inner square - 80-3 = 77

4) outer square - 60+4 = 64
inner square - 60-4 = 56

5) outer square - 30+8 = 38
inner square - 30-8 = 22

6) outer square - 24+10 = 34
inner square - 24-10 = 14

7) outer square - 20+12 = 32
inner square - 20-12 = 8

8) outer square - 16+15 = 31
inner square - 16-15 = 1