A stall's daily sales for one week, in rupees hundred, were Monday 120, Tuesday 150, Wednesday 90, Thursday 180, Friday 210, Saturday 240, Sunday 60. A three-day moving average is found by averaging each day with the day before it and the day after it. What is the three-day moving average centred on Friday?

Centred on Friday means Thursday, Friday and Saturday, so the average is (180 + 210 + 240)/3 = 630/3 = 210. The point of a moving average is to smooth a jumpy series without letting one odd day dominate, and here it happens to land on Friday's own figure because Thursday is as far below Friday as Saturday is above it. The value 160 comes from the three days ENDING at Friday, that is Wednesday, Thursday and Friday: (90 + 180 + 210)/3 = 160 — a trailing average, not a centred one. The value 150 is the mean of the whole week, 1050 ÷ 7, which smooths away the very variation the moving average is meant to display. The value 225 averages Friday with Saturday alone and leaves Thursday out.