A vendor in Surat sold goods worth 648 rupees on Monday, 275 rupees on Tuesday and 352 rupees on Wednesday. Grouping the two amounts that make a round number first, what were his total sales?

Natural numbers (1, 2, 3, …) answer 'how many?' when there is at least one of something. But what if there are none? You need a number for that — and that number is zero. Adding 0 to the natural numbers gives the whole numbers: 0, 1, 2, 3, …. Every whole number sits at a fixed point on the number line, and they never run out — for any whole number you pick, there is always a larger one.