arithmatic operations will be purly algorithum part… first i think you should start interfacing LCD and keypad as you don’t know that part…
here i am sending you the LCD and keypad interfaicng tutorial links….
LCD
http://www.engineersgarage.com/microcontroller/8051projects/interface-lcd-at89c51-circuit
Keypad
http://www.engineersgarage.com/microcontroller/8051projects/interface-keypad-AT89C51-circuit
as soon you cover these let know your algo we try to create a good algo first then we will go for the code…
and Kaubstubh has post that link since it is a combination of LCD and keypad interfaicng with 8051…..