Is 30/- of petrol plus 70/- equals 100/- of petrol. Is **30/-** of petrol plus **70/-** equals **100/-** of petrol.The worker at petrol pump filled me 30/- of petrol,then I said I wanted **100/-** of petrol.Then he said,"Oh sorry!Let me fill 70/- of petrol".Is that equals to 100/- of petrol.Am I being thugged or its just a mistake done by the worker?By the way petrol is **65/-per litres** here.

If petrol is 65/- per liter, then it is the same as saying that for 1/- you will get 1/65 liters of petrol. For $x$/- you will get $x \times 1/65$ liters of petrol. So $30 \times 1/65 + 70\times 1/65 = 100 \times 1/65$ which is 100/- worth of petrol.

The above method is valid for any rate for petrol: for example, if petrol is 70/- per liter, replace 65 with 70 everywhere. In fact, it is valid for any sort of item: for example, if one car is for 100/-, then 1000/- of cars is the same as 600/- of cars plus 400/- of cars.

Money is defined as a unit of value, which means that even though you can't add apples and oranges, you can add 10/- of apples and 20/- of oranges to get 30/- of fruits in total. The same works for your question.