Question 1207335
Mark bought a number of pens at an average cost of $2.50 He bought another pen for $8.50 and the average cost of the pens became $4.50. How many pens did he buy altogether.


Let Mark buy x pens
Average cost of pen $2.50

Total paid by him for  pens 2.50x

Another pen he bought for $8.50
Total cost of all pens = 2.50x +8.50

The average cost now became  $4.50

Average cost of all pens = total purchase cost /Total number of pens

4.50 = (2.50x+8.50)/(x+1)

4.50*(x+1)= 2.50x +8.50

4.50x +4.50 = 2.50x +8.50

4.50x-2.50x = 8.50-4.50

2x = 4

x=2

Altogether he bought 3 pens.

Check
2*2.50 +8.50=13.50

number of pens 3

13.50/3 = 4.50