Aman parked his car in a carpark at 08 30 and retrieved it at 15 45 on the same day.
if the parking charges are $1.50 for the first hour and 80 cents for each subsequent half hour or part thereof, how much must he pay for parking his car there?

First, we need to work out how long he actually parks his car for.
To make this easier, we will work in steps.
First, we want to get to the next hour
From 8:30 to 9:00, we add 30 mins
From 9:00 to 15:00, we add 6 hours
From 15:00 to 15:45, we add 45 mins

Next, we need to add up all the times
6 hours + 30 mins + 45 mins = 7 hours and 15 mins

As he cannot pay for 15 minutes, we must round up to the nearest half hour
He pays for 7 hours and 30 minutes

The first hour is $1.50, therefore we take away this first hour from the total to work out how many 80 cents units there are

6 hours and 30 minutes
If 2 x 30 = 60, we must multiply the number of hours by 2 and then add 1 to find out how many units there are
Alternatively, we could divide 6 hours and 30 minutes by 30 minutes - however this can get confusing due to the change of units.
2 x 6 = 12
12 + 1 = 13
This is how many half hours there are
Multiply this by $0.80
0.8 x 13 = $10.40
Finally add this to the cost of the first hour

$10.40 + $1.50 = $11.90

Num of plants = (original#ofplants)*(1.6)^t