When solving pie charts in Data Interpretation remember the following things.
a) Look for all the information and understand the relationships of the information provided with the pie chart as much as possible. Understanding the information provided will help you solve the pie chart very easily.
b) The percentages (or ratios) given are not equal to the actual number. The actual number for each entity should be derived by multiplying with the total units present (if percentage is given). This is a 'silly mistake' committed by many students and it should be avoided.
c) Try approximation when applicable. In questions like these which are based on fractions and ratios, it is easy to reject incorrect answers very fast and this will help you in improving your speed. Grasp the relative sizes of each entity in the pie chart and this will help you easily reject a few answers. Using approximation, try to get the correct answer fast.