Instruction 18-4
a Manufacturer of Computer Disks Took Samples of 240

Instruction 18-4
A manufacturer of computer disks took samples of 240 disks on 15 consecutive days. The number of disks with bad sectors was determined for each of these samples. The results are in the table that follows.
$$\begin{array} { l l l } \text { Day } & \text { Bad } & \text { \% Bad } \\1 & 9 & 0.037500 \\2 & 7 & 0.029167 \\3 & 4 & 0.016667 \\4 & 6 & 0.025000 \\5 & 8 & 0.033333 \\6 & 3 & 0.012500 \\7 & 6 & 0.025000 \\8 & 10 & 0.041667 \\9 & 16 & 0.066667 \\10 & 24 & 0.100000 \\11 & 15 & 0.062500 \\12 & 9 & 0.037500 \\13 & 4 & 0.016667 \\14 & 8 & 0.033333 \\15 & 6 & 0.025000\end{array}$$
-Referring to Instruction 18-4,a p control chart is to be made for these data.The centre line of the control chart is _______.