Long distance phone call

The way I would solve this question is break the pieces down into separate parts:

Part 1: The first 10 minutes will cost $2.50
Part 2: The next 10 minutes will cost $1.50

We have now used up 20 minutes which cost us $4.00

The remaining 55 minutes will be charged at $0.10 per 20 seconds. We have 55*3 = 165 such segments.
165 * $0.10 = $16.50

[spoiler]$2.50 + $1.50 + $16.50 = $20.50[/spoiler]

first 10 min = 2.5$
next 10 min = 1.5$
0.10 for 20 seconds

75 minutes = 10min + 10min + 55mins

=> 2.5 + 1.5 + 55*3*(0.10)
=> 4 + 16.5
=> 20.5$

[spoiler]{D}[/spoiler]