Let's set up a proportion.
3/20=x/100
Because 20*5=100, you can determine x by multiplying the numerator by 5 as well.
3*5=15
x=15
15%
<h3>Given</h3>
burned : unburned = 1 : 3
<h3>Find</h3>
burned/total as a percentage
<h3>Solution</h3>
burned : total = burned : (burned + unburned) = 1 : (1+3) = 1 : 4
As a fraction, this is 1/4 = 0.25 = 25%
25% of the total cookies were burned.
_____
"%" is a fancy way to write "/100". You know from place value that 0.25 (twenty-five hundredths) is 25/100, so that can also be written as 25%.
Remember that there are always 360 degrees in a full circle. Since 
(it is a right angle), we can find:
Substituting in the values provided in the problem into our equation gives us:
Given this, we can find the 
:
