Question 836583
The n-th unit costs $5n+5. Also, the cost for 5 units is 10+15+...+30 = $100, but be careful; the sixth unit costs $35, not $105.


For n >= 1, the cost for n units is

*[tex \Large 10 + 15 + \ldots + 5n+5]


This is one of those "Gauss sums," and the sum is *[tex \Large \frac{(5n+15)n}{2} = \frac{5n(n+3)}{2}].