What is Division?

The division $20 \div 4$ answers two kinds of questions:

• sharing: $20$ marbles shared among $4$ children — how many marbles each? ($5$ each);
• grouping: $20$ marbles put in bags of $4$ — how many bags? ($5$ bags).

Both questions have the same answer, because both undo the same multiplication: $4 \times 5 = 20$.