So, as Gilbert bought 3 shirts -- we multiple $15.90 by 3 to get us, $47.70. However, as we need to include tax (20%) we find 20% of that cost. 10% of 47.70 is 4.77 -- we then double that as we are calculating 20%. That gives us 9.54 which we add onto the previous cost of the shirts: 47.70 + 9.54 = 57.24 (ANSWER)