Total cost of fans = ($1716 + $156) ÷ 2 = $936

Total cost of air purifiers = $1716 – $936 = $780 (a)

 “There are 4 times as many fans as air purifiers.”

Let’s make the no. of air purifiers the same as fans,

Total cost of air purifiers = 4 x $780 = $3120

Diff in the total cost = $3120 – $936 = $2184

Diff in individual cost = $182

No. of fans = $2184 ÷ $182 = 12 (b)

Hope you understand and good luck with your exams! :))

Thank you very much your for clear explaination