$($b$)$ four capacitances of $25\backslash$mu F each are connected in parallel to act as a single capacitance of $100\backslash$mu Frequired for the successful operation of a unit. If the reliabilities of these capacitors are $0\mathrm{.}6,0\mathrm{.}7,0\mathrm{.}8,$ and $0\mathrm{.}9$ respectively, find the reliability of the unit.