Answer:
From the information, we know that the plate will hold 3 pieces of sushi, each with a diameter of 1 1/2 inches, and there be 1/2 inch space from the edge of the of the suchi to the edge of the plate.
The diamter of the suchi plates we need to puchase should be the sum of the diameter of the sushi and the the space between them and the edge of the plate:
(3 . 1 1/2) + (2 . 1/2)
= (3 . 3/2) + 1
= 9/2 + 1
= 9/2 + 2/2
= 11/2 = 5.5 (inches)