Using HP 10bII for Cost Estimation with Linear Regression
Johnson’s Chair Company has experienced the following costs for the first 6 months of the year:  
# Chairs Made 5,000 5,500 4,800 5,300 4,950 5,150 
Total
Costs $120,000 $122,100 $118,540 $122,400 $119,100 $124,200 
Entering data in HP 10bII:

[Shift] [C All]

5000 [Input] 120,000 [E+]

5,500 [Input] 122,100 [E+]

4,800 [Input] 118,540 [E+]

5,300 [Input] 122,400 [E+]

4,950 [Input] 119,100 [E+]

5,150 [Input] 124,200 [E+]
Johnson’s fixed cost:

Press 0 then [Shift] [y,m]

Display 89,449.381 is the Answer
Johnson’s variable cost (cost to build on chair):

Press 1 then [Shift] [y,m]

Display 89,455.559

Deduct this with the above fixed cost 89,449.381

6.178 which is the cost for one unit of chair.

or Press [Shift] [Swap] to get the same answer
How good is the fit of the linear regression line generated?

Press 0 then [Shift] [x,r]

Display 14,480.29

[Shift] [Swap]

Display 0.716 and this is the answer
Total costs for 5,400 chairs:

Enter 5,400 then [Shift] [y,m]

Display 122,806.907 is the Answer.
Total costs were $125,000, how many chairs:

Enter 125,000 then [Shift] [x,r]

Display 5,755.023 is the Answer.
This solution is also applicable to HP 10b Financial Calculator.