Question

during a festive season a shop offered a discount of 20 percent on the marked price of all items find the SP of 3 trousers mardep at rs 1800 each and two t shirts marked at rs 550 each

Answer 1

Think of the retail price as the whole price or 100%. Since the items are discounted 20% you end up paying 80% of the retail price for each item. 80% of the retail price is the sale price.

TROUSERS
We need to find 80% of 1800. Percent means out of 100 so we can represent it using $\frac{80}{100}$ or .80 whichever is easier for you to work with. Multiply this by 1800 and you have your sales price for 1 trouser. That is, the sales price for a trouser is (1800)(.8) = 1440.

SHIRTS
Same as we did for trousers we multiply the price of a shirt (550) by (.8) to get the sale price. It is 440.

3 TROUSERS AND 2 SHIRTS
The sales price for 3 trousers and 2 shirts is (3)(1440) + (440)(2) = 5200

NOTE
I noticed that you didn't include currency (dollars, pounds, euros, for example). If these are dollars then the price of a trouser might be $18.00 as opposed to 1800 which seems really high. Similarly I would expect 18,00 Euros as opposed to 1800 Euros for a pair of pants. If this is the case take the answers given above and merely insert a decimal point to spaces from the left (or in Europe and other parts of the world a comma).