Question 955278
Add their rates of filling:
{{{ 1/12 + 1/20 = 1/t }}}
where {{{ t }}} = time in minutes
Multiply both sides by {{{ 60t }}}
{{{ 5t + 3t = 60 }}}
{{{ 8t = 60 }}}
{{{ t = 7.5 }}}
It takes 7.5 min with both taps turned on