Question 906942
Northern's method can be formulated.
(12x20) + (9x20) = 420 
(12x40) + (9x40) = 840 
(12x30) + (9x30) = 630 
(12x32) + (9x32) = 672 
So it will take 32 minutes

(12*t)+(9*t)=f
factor
t*(9+12)=f
add
t*21=f where f=672
substitute
t*21=672
solve
t=672/21
t=32 
which is essentially what tutor josmiceli said in a more complicated way.