Question 916524
rate for A alone 20
rate for A & B together 15
1/a+1/b=1/x
1/20+1/b=1/15
1/b=1/15-1/20
1/b=20/300-15/300
1/b=5/300
1/b=1/60
b=60
rate for B alone 60
check
1/20+1/60=1/15
60/1200+20/1200=1/15
80/1200=1/15
ok
codewa

ewatrrr  original mistake was

3x + 60 = 5x
which should have been 
3x + 60 = 4x
then the next step is 60=x

ewatrrr own check is wrong

Her check should be 
{{{3/60 + 2/60 = 5/60}}}
{{{3/60 + 2/60 = 1/12}}}
not 
{{{3/60 + 2/60 = 1/15}}}