21/51 as a percentage is 41.18%. This value is the rounded result you get when you compare the part (21) to the whole (51).
The fraction 21/51 can be reduced to a simpler form, but the percentage stays the same. Knowing the percentage helps you see at a glance how large the part is relative to the whole.
When you need a quick estimate, thinking of 41.18% tells you that the part is just under half of the whole. This makes it easy to compare with other fractions or percentages in everyday situations.
