Joshua has a stored value card with $100 to buy food at a seafood restaurant. If he
orders 1 kg of chilli crab and 4 tiger prawns,
there will be $12 left in the value of the card.
If he orders 2 kg of chilli crab and 2 tiger
prawns, he needs to pay $28 more. How
much is left in the card or how much will he
need to pay if he orders 1.5 kg of chilli crab
and 2 tiger prawns?

no money left (0$), but no extra payment either.

Step-by-step 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.