The Morgans paid $50.40 for 1500 kilowatts hours of electricity one month and $50.80 for 1680 kwh the next month how much should they expect to pay for 1840 kwh?

We use the data to determine a line: 
 

and using the first pair of data we can write: 

 

Now when y=1840 we get 

 


When we solve for x we get x=$51.16 

:)