To get this, you need to do 8/10 and make it over 100, which is 80/100. Then, you turn this number into a percent by just dropping the 100 and adding a % to 80 (it's this easy if the bottom denominator is 100)
The work should look something like this;
8:10 8/10 80/100 80%
So, there is an 80% chance that the crayon you pick will be yellow. Any questions?