How to solve

A process engineer performed jar tests for a water in order to determine the optimal $pH$ and dose using alum. A test was conducted by first dosing each jar with the same alum dose of $10m\frac{g}{L}$ and varying $pH$ in each jar from $5\mathrm{.}0$ to $7\mathrm{.}5$ with an increment of $0\mathrm{.}5$ unit of $pH.$ After the first test, he$/$she plotted the results of remaining turbidity versus $pH$ and found the optimal $pH$ was $6\mathrm{.}25.$ He$/$she continuously perform a second set of jar tests by holding the optimal $pH$ of $6\mathrm{.}25$ constant and varying alum doses from $10$ to $15m\frac{g}{L}.$ Here are the results:

Table $1.$ Results of Jar Tests for raw water at optimal $pH$ of $6\mathrm{.}25$

Turbidity of raw water $=15$ NTU

Alkalinity of raw water $=5m\frac{g}{L}$ expressed as CaCO$3.$

$\backslash$table$[[\backslash$table$[[$Alum Dose$],[($mg$/)$