Click friends For a market research, a questionnaire on the use of three devices is submitted to the students of a class: computer, tablet, smartphone. The answers show that all the children use at least one tool, namely: 18 children have computers; 19 the tablet; 13 the smartphone; 11 the computer and the tablet; 8 the tablet and smartphone; 9 the computer and smartphone; 7 all three appliances. Determine how many students have: to. the smartphone and the computer, but not the tablet; b. only the smartphone. c. How many students are in the class?