Instruction 18-8
Below Is the Number of Defective Items from a Production

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