prospectors are drilling for oil. The cost of drilling to a depth of 50m is £500. To drill a further 50m costs £640 and, hence, the total cost of drilling to a depth of 100m is £1140. Each subsequent extra depth of 50m costs £140 more to drill than the previous 50m.

the total sum of money available for drilling is £76000. Find, to the nearest 50m, the freatest depth that can be drilled.

a=500 d=140 i'm not sure what n=....