$12\mathrm{.}5\mathrm{.}2($a$)$ Calculate the celerity of a pressure wave in open fresh water at $\backslash (20^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash ).$

$($b$)$ What will the celerity be if the water is contained in a cast$-$iron pipe with $\backslash (489-\backslash$mathrm$\{$mm$\}\backslash )$ inside diameter and $\backslash (14\mathrm{.}3-\backslash$mathrm$\{$mm$\}\backslash )$ wall thickness? $($c$)$ Express the celerity in the pipe as a percentage of the celerity in the open water.