You can convert this question into a fraction: 4/5 where 4 = the cookies Will ate and 5= the cookies that were on the plate.
Simple Answer: You just convert 4/5 into a decimal, which is .8 or 80%
Complicated Answer: If you convert this into a percentage, you can either use a proportion: 4/5=x/100 and then cross multiply, which gives you 5x=400. After that, we divide both sides by 5, thus getting rid of the 5 next to the x, and you're left with x=80. So, 4/5 is equal to 80%.
Hope this helped. :)