At a restaurant, Toby knew he would tip his server 20% of whatever the cost (c) of the final bill was. He knew he could use the strategy c + 0.2c to calculate his final cost after paying the 20% tip. Which other process could Toby use to calculate his cost after tip?
A: c + 20
B: 1.2c
C: 0.8c
D: c/0.2c