The Weekly Warlock Wind-Up: Shopping (answer)

This week, I gave you this:

“At a shop, the first 15 customers of the day spent an average of £3 each. After a further 30 customers the average amount rose to £9. What was the average amount spent by the last 30 customers only?”

Disputes about the use of Pounds Sterling aside, you all apparently got on quite well with this problem. But what was the answer, and how to get there? See for yourself after the break.

So first, we can see that if the first 15 customers spent an average of £3, the total amount spent by the first 15 only must therefore be 15 * 3 = £45.

Next, we need to look at the total amount for all 45 customers (15 +30). Since the average was £9, the total overall amount must have been 45 * 9 = £405.

To get the total amount spent by the last 30 customers, we can take the total for the first 15 away from the overall total, giving us 405 – 45 = £360.

Finally, we can get the average for the last 30 by dividing their total by the number of people: 360 / 30 = £12. So their average is £12.

See you on Monday for the next puzzle!

Vel.