X= Z-Y(n+1)
Y is the number of people in the household who don't actively cycle.
So, the total number of people in the house, less the number who don't actively cycle, multiplied by n+1 which here has its accepted meaning, gives a notional ideal number of cycles in the household.
Reality chez Cube is 5 bikes in total. One child is content with the number of bikes she has, one is semi-content with his current bike but occasionally hankers after a big hit susser, so that would be n+0.5 and one has two bikes he uses regularly, plus one he wants to sell, so that he can fund the next project, which is n+1, or would be if n-1 would hurry up and sell, so must be written as n+2.
I think.