If this is difficult to understand, you can also picture it in this way. For every Figure X, there will be X number of squares on each flap. And in the middle, it will always be a shaded square.
Eg on Figure 4, there are 4 squares on each flap. And on each flap, there will be an equal split of shaded vs unshaded squares. So you will need to divide X into 2. On Figure 20, there are 20 squares on each flap, 10 shaded and 10 unshaded. You can count the number of shaded squares on Figure 20 as 10+10+10+10+1 (the one in the middle) = 41.