A half adder is implemented with XOR and AND gates. A full adder is implemented with two half adders and one OR gate. The propagation delay of an XOR gate is twice that of an AND$/$OR gate. The propagation delay of an AND$/$OR gate is $1\mathrm{.}2$ microseconds. A $4-$bit ripple carry binary adder is implemented by using four full adders. The total propagation time of this $4-$bit ripple carry binary adder in microseconds is$?$