In a class of 24 students, 22 have a mobile phone and 10 have a portable DVD player.One student has neither.
- How many students have:
- a) a mobile phone only;
- b) a portable DVD player only;
- c) both?
From the numbers of students given in the question we can see that some students definitely have more than one device; 23 students own devices and there 32 devices. Therefore, nine must own both types of device (that answers part c), 13 must own only a mobile phone (part a) and one has just a DVD player (part b).
Notice in this question I didn't answer the questions in order. Perfectly valid and sometimes a good technique to overcome difficulties with a question.