Question 170779
they each fill some FRACTION of the tank, based on their individual filling times


let x=remaining fill time __ {{{3/40+6/20+(x+6)/30=1}}}


multiplying by 120 __ 9+36+4x+24=120 __ 4x+69=120


subtracting 69 __ 4x=51


dividing by 4 __ x=51/4 __ x=12min 45sec