Let 
be the number of rides Chandler takes in a month. Then the cost with the MetroCard is still $81, but the cost without the MetroCard is 
. So we can set up an equation representing what we want: "The cost with a MetroCard of r rides in a month is less than the cost without a MetroCard." In equations,
Thus, at a minimum, Chandler must take 41 rides for his MetroCard to be cheaper than not having it.
Thus, at a minimum, Chandler must take 41 rides for his MetroCard to be cheaper than not having it.