For example we may want to know how much of our weekly work schedule is spent with certain types of clients, and we'd like to know it as a percent.
The question may look something like this:
Job Performance Appraisal Sample Question
Now, for the respondent this question actually involves 2 seperate cognitive tasks, which results in lesser quality data for you (and more frustration for them).
Creating this survey question in web format may reduce some of the frustration, as survey platforms can automatically calculate the total percent for respondents and notify them when the total they reported amounts to greater than 100%. While this is a great practical advancement that makes it slightly easier to get the answers we need and provides cleaner data, regardless, adding a math problem (calculating percentages) to a question(s) will reduce the construct validity of your question.
Whether we use paper, phone or a web program that helps the survey respondent total to 100%, surveyors should pause before putting too much faith in responses received from complex question types.