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

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 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).