C - how do I increase int a if int b increases by a set amount?

Here is my question below.

In a certain country, the cost to post a parcel depends on the weight of the parcel. It always costs $3.50 to post a parcel that weighs 2.0kg or less. There is an additional cost of $1.50 for every kg (or part of a kg) over 2.0. Write a C program that reads in the weight of a parcel and calculates the cost to post it. The output of your program should be similar to the examples below.

thanks for the help :)