Answer:
no money left (0$), but no extra payment either.
Explanation:
x = price per kg of crab
y = price per prawn
if he orders the first list, we know $12 will be left on his card, that means the order costs 100 - 12 = $88.
1×x + 4×y = 88
for the second list we know he would have to pay $28 extra, in addition to the $100 in the card.
so, 100 + 28 = $128
2×x + 2×y = 128
x + y = 64
now we have a system of 2 equations with 2 variables. we use one equation to express one variable by the other, and that we use then in the second equation to solve it.
x = 64 - y
(64 - y) + 4y = 88
64 + 3y = 88
3y = 24
y = 24/3 = $8
=>
x = 64 - y = 64 - 8 = $56
so, if he orders 1.5 kg of crab and 2 prawns we get a cost
1.5×56 + 2×8 = 84 + 16 = $100
so, he will not have any money left in the card, but he will not have to pay anything extra either.