The tar yield of cigarettes is often assayed by the following method: A motorized smoking machine takes a two-second puff once every minute until a fixed butt length remains. The total tar yield is determined by laboratory analysis of the pool of smoke taken by the machine. Of course, the process is repeated on several cigarettes of a brand to determine the average tar yield. Given here are the data of average tar yield and the average number of puffs for six brands of filter cigarettes.

Average tar (milligrams) 12.2 14.3 15.7 12.6 13.5 14.0 Average no. 8.5 9.9 10.7 9.0 9.3 9.5 of puffs

(a) Plot the scatter diagram.

(b) Calculate r.

Remark: Fewer puffs taken by the smoking machine mean a faster burn time. The amount of tar inhaled by a human smoker depends largely on how often the smoker puffs.