how do i do something like this

I am completely new at this and would like to know how i can input say A1= number of boxes, and A2= number of pieces, and A3= sq. ft.. So 1 box would have 5.5 pieces equaling 11.72 sq. ft. Yeah no problem right? How can I make it to when i change the number in any cell, such as sq. ft. goes down, and it will reflect in pieces and boxes. I guess the sq. ft. per box and pieces are all true being that 1 box= 5.5pcs or 11.72 sq.ft. but i cant find anything to show me how to write the formula that keeps each beginning number true. At least that I can understand. Any help would be appreciated