Question 892875
Let r = semi skilled worker rate for 1 worker
Let R = skilled worker rate for 1 worker


{{{4R*5=1}}} and {{{5r*6=1}}}


{{{R=1/20}}} and {{{r=1/30}}}, you know the one-worker work rate as job per time in days.


The rate of a group of workers together is the combined rate by addition of all of the workers in the group.


Your question needs a time variable, d for days.
1 semi skilled plus 2 skilled workers;
{{{highlight_green((r+2R)d=1)}}}
{{{d=1/(r+2R)}}}
{{{d=1/(1/30+2/20)}}
{{{d=1/(1/30+1/10)}}}
{{{d=1/(1/30+3/30)}}}
{{{d=1/(4/30)=30/4}}}
{{{d=15/2}}}
{{{highlight(d=7&1/2)}}} days