this is a problem of two equation and two unknowns,
let x be the price of the plates
and y be the price of the cups
for the first cook, his total purchase price is $ 21.50, in equation form:
6x + 10y = 21.50
for the second cook, his total purchase price is $ 18.50, in eqution
9x + 4y = 18.50
solve the 2 equations simulataneously
x = $ 1.5 per plate
y = $ 1.25 per cup
are the prices of the two items