Right choice is (d) Total length of four wall – 20 m, length of short wall – 6.8 m
The best I can explain: Total length is (long wall) 6.80. So, centre to centre length 6.80-80 = 6, 2 wall so 6+6 = 12.
Total length of short wall 4.80. So, centre to centre length 4.80-80 = 4, 2 wall so 4+4 = 8. Total length of center to centre 4 wall = 20 m.
Length of short wall in-to-in is 4.80-(80+80) = 3.20 m.