Observation

The Chief Examiner reported that very few candidates attempted this question and their performance on this question was poor.

In part (a), they were expected to draw the graph as shown:

In part (b), they were expected to let  . Then substituting into sum of A.P formula gives .
Simplifying gives . Solving gives . Substituting into the sum the first 20 terms gives  =10[13016+2698] .
Amount left is  gives N 472,560157,140 = N 315, 420.00.