70 students at school were asked to complete a survey about the location of their next field trip. 28 students choose zoo and 33 students choose the amusement park.The remaining students did not complete the survey. What is the percent of survey participants who choose the zoo.
Since the question only pertains to those who actually participated in the survery, you can add upp the two groups and then divide the number of people who choose zoo by the total number of participants. Multiply the answer by one hundred to get the percentage rather than fraction (this isn't necesarrily required just easier to read out for most)
If the question was how many people chose zoo out of all the people, even those who didn't participate you simply do 100*(28/70)=40%