GENERATE A FUNCTION THAT MODELS THE SITUATION. Consider the type of function described—it is not linear!

A shipping company charges $4 for the first pound and $3 for each additional pound or part thereof.
As you try to come up with the equation, consider the following:
-How much should you be charged if your shipment weighs 0.7 pounds? $4
-How much should you be charged if your shipment weighs 1.0 pounds? $4
-How much should you be charged if your shipment weighs 1.3 pounds? $7
-How much should you be charged if your shipment weighs 1.8 pounds? $7
-How much should you be charged if your shipment weighs 2.0 pounds? $7
-How much should you be charged if your shipment weighs 2.1 pounds? $10
Make sure that your function gives the correct answer for each of these.
Can someone pease help, all I could figure out was that the function uses linear programming and that its a step-wise function too. But I don't know how to generate the function for this. I asked before but MathMe only said how to graph it which I already know. Thanks for the help. :)