3 red hats, 2 yellow hats, 4 blue hats, 6 green hats, 5 orange hats

• 3 of his hats are red. (3)
• He has 1 fewer yellow hat than red hats. (3-1 = 2)
• He has 1 more blue hat than red hats. (3+1 = 4)
• To find the number of green hats, add the number of yellow hats to the number of blue hats. (2+4 = 6)
• The rest of his hats are orange. (3+2+4+6 = 15; 20-15= 5)