Answer:
20 percent
Explanation:
Total number of pieces in a puzzle = 500
No. of pieces in first container = 220
No. of pieces in second container = 180
Let no. of pieces in the third container be x.
We get,

On adding 220 and 180, we get

On transposing 400 to RHS, we get

Percentage of pieces in the third container = (no. of pieces in third container/total no. of pieces in a puzzle)


Therefore, percentage of pieces in the third container = 20 percent