Instruction 18-6
Below Is the Number of Defective Items from a Production

Instruction 18-6
Below is the number of defective items from a production line over 20 consecutive morning shifts.
$$\begin{array} { | l | l | l | l| } \hline \text { Day } & \text { Nonconf. } & \text { Day } & \text { Nonconf. } \\\hline 1 & 26 & 11 & 22 \\\hline 2 & 27 & 12 & 26 \\\hline 3 & 23 & 13 & 21 \\\hline 4 & 21 & 14 & 22 \\\hline 5 & 26 & 15 & 19 \\\hline 6 & 20 & 16 & 17 \\\hline 7 & 35 & 17 & 21 \\\hline 8 & 30 & 18 & 18 \\\hline 9 & 21 & 19 & 16 \\\hline 10 & 25 & 20 & 17\\\hline\end{array}$$ Note: Nonconf. = Nonconformances
-Referring to Instruction 18-6,a c chart is to be constructed for the number of defective items.The centre line of this c chart is located at _______.