The cost of dinner for a group of students is partly constant and partly varies directly as the number of students. If the cost is ₦74.00 for 20 students and ₦96.00 for 30 students, find the cost for 15 students.
Worked solution (try it first)
Partly constant and partly varying as the number n: C=a+bn.
a+20b=74 and a+30b=96.
Subtract: 10b=22, so b=2.2.
Then 20b=44, so a=74−44=30.
For 15 students: 2.2×15=33, so C=30+33=63, which is ₦63.00, option B.