Question 1001833
Helping here for #2, and you should try to work through #1 yourself.


One man rate is r.
The first sentence gives  {{{20r*24=1}}}, for the 1 job to be done.
The second sentence uses an unknown amount of time, t, after which men are added (4 of them) to finish the job in 3 days.  This work was split into two parts, this way:


{{{highlight_green(24r*t+(24+4)r*3=1)}}}.
If all of this is well understood, then you can continue.  The question asks for the value of t.  The value for rate r will come from the equation from the first sentence.


System of equations if this is more comfortable to see,
{{{system(20r*24=1,24rt+28r*3=1)}}}