Calculate Stt, Smm and Stm.As part of a statistics project, Gill collected data relating to the length of time, to the

nearest minute, spent by shoppers in a supermarket and the amount of money they spent.

Her data for a random sample of 10 shoppers are summarised in the table below, where t

represents time and £m the amount spent over £20.

(You may use Σt2 = 5478 Σm2 = 2101 Σtm = 2485)

heres the table, I know how to calculate the above sums, but what I cant understand is how to get the sum of £m ?

from the table would I not just add all the values under m? I get £216, but this is incorrect?

the correct sum for £m = 61 and I have no idea how to get that,;;

why is this not correct?

Also I have taken into account the minus signs since m stands for the amount over £20, than -3 is 20-3 = £17

right?