Archana sells $12kg$ of prawns for $₹1200$ and $10kgs$ of fish for $₹800$. If Sushma wants to buy $3kgs$ of prawns and $4kgs$ of fish, how much does she spend?
 

$12kgs$ of prawns$=₹1200$
$10kgs$ of fish $=₹800$

The price of $1kg$ prawn  $=\frac{1200}{12}=₹100$

The price of $1kg$ of fish$=\frac{800}{10}=₹80$
To buy $3kgs$ prawns  $=3\times 100=₹300$

 To buy $4kgs$ fish $=4\times 80=₹320$

The total amount  $=300+320=₹620$
The correct option is (D).