The following table shows the number of clients who attended a particular accountancy practice over the last four weeks and the total costs incurred during each of the weeks:
Week Number of clients Total cost
$
1 400 36,880
2 440 39,840
3 420 36,800
4 460 40,000
Applying the high low method to the above information, which of the following could be used to forecast total cost ($) from the number of clients expected to attend (where x = the expected number of clients)?