AWNSER:
40%
EXPPLENATION:
1. you need to find how many marbles are in the bag in total. e.g.(20)
2. find out how many of the marbles are blue and put it into a fraction blue/total number of marbles. e.g.(8/20)
3.multiply the fraction so that you have 100 as your denominator since 'per-cent' means out of 100. e.g.(8/20 x 5= 40/100)
4. remove the fraction/ take off the 100 and your done
:D