$(10\%)2.$ A concrete mixture was designed for a $28-$day characteristic strength of $20$MPa with an anticipated $($expected$)$ standard deviation of $4$MPa and $10\%$ failures permitted $)=(1\mathrm{.}28.$ The first $10$ specimens test results were as follows: $26\mathrm{.}3,26\mathrm{.}1,29\mathrm{.}1,20\mathrm{.}1,24\mathrm{.}5,21\mathrm{.}3,22\mathrm{.}5,31\mathrm{.}4,28\mathrm{.}1,29\mathrm{.}0(MPa)$

Calculate the standard deviation and coefficient of variation of test results. Assuming that a $10k\frac{g}{m^3}$ change in cement content produces a change in mean $($average$)$ strength of $1\mathrm{.}2$MPa, suggest how the cement content of the mix could be altered $($changed$).$ Use your calculator for determining $S$ and $$ or use the following expressions:

$S=\sqrt[\phantom{2}]{\frac{(x-{)}^2}{n-1}},v=\frac{s}{}100,f_{cm}=f_{ck}+ts$