June 2012 Question no 3a

Why did the examiner calculate the Average trend of the centre moving like this: (1287.5-1068.75)/5= 43,750 units?
is it the standard formula? if not, why is it divided 5? and why using only the first and last value of the Moving Average?

Please could you tell me the alternative way of arriving at the same answer?

Other part of question i understood.

Thanks,

Ogo