How to factor just the mean without the weight

Question

The data below represents the number of T-shirts sold per week by a student who started his own online T-shirt business. Find the mean number of T-shirts sold per week. (Round your answer to the nearest tenth if necessary.)

Tshirts sold per week Frequency

5 4

10 5

15 2

20 6

I come up with 50/4=12.5 but the course work says 12.2

How?

Answer 1

Mean = total T shirts sold / total weeks

= [(5x4) + (10x5) + (15x2) + (20x6)] / (4+5+2+6)

= (20+50+30+120) / 17

= 220/17

= 12.9