Thread: calculating sum

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.

Calculate Stt, Smm and Stm.
(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?

Originally Posted by Tweety

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

If you work like this then maybe ,as -3 is 20-3, 17 is 20+17=37 ??? If i understand well the last line of your exercise because english isn't my mother language

Are you sure the correct answer is 61?

yes certain that the right answer is 61,

Heres the answer book;

Originally Posted by Tweety

Calculate Stt, Smm and Stm.
(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?

Where did you get those numbers? In the table you show m= 17 only occurs once,not twice. There is no "1" or "4" in the table, etc.

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

right?

Originally Posted by HallsofIvy

Where did you get those numbers? In the table you show m= 17 only occurs once,not twice. There is no "1" or "4" in the table, etc.

since m stand for the amount spent over £20, I thought the minus sign means you have to subtract the amount from 20, but apparently not,

you just add up the numbers including the minus signs which = 61

thanks.