Question 948826
You made a mistake.
A working for 4 days would be,
{{{A*4=(1/12)4=1/3}}}
So that leaves {{{2/3}}} to complete.
.
.
.
So then use the rate for A and B together.
{{{A+B=1/12+1/20=5/60+3/60=8/60=2/15}}}
So then,
{{{(A+B)t=2/3}}}
{{{(2/15)t=2/3}}}
{{{t=5}}} {{{days}}}