Using the percentage concept, it is found that he keeps 21% of his mask collection on the wall.
What is a percentage?
The percentage of an amount a over a total amount b is given by a multiplied by 100% and divided by b, that is:
[tex]P = \frac{a}{b} \times 100\%[/tex]
In this problem, the parameters are a = 63 and b = 300, hence the percentage is given by:
[tex]P = \frac{63}{300} \times 100\% = 21\%[/tex]
More can be learned about the percentage concept at https://brainly.com/question/10491646
