The two laws of motion are
[tex]\text{Low:}\ d=v_lt\\\text{Sim:}\ d=v_st[/tex]
Since Mr Sim's speed is 15 km/h less that Mr Low's, we have
[tex]\text{Low:}\ d=v_lt\\\text{Sim:}\ d=(v_l-15)t[/tex]
Note that the distance d is the same for both equations, because they traveled the same path.
20 minutes is 1/3 of a hour, so at t=1/3 Mr Low traveled the whole distance, while at the same time Mr Sim traveled 4/5 of the distance:
[tex]\text{Low:}\ d=\dfrac{v_l}{3}\\\text{Sim:}\ \dfrac{4d}{5}=\dfrac{(v_l-15)}{3}[/tex]
If we multiply Sim's equation by 5/4, we have
[tex]\text{Low:}\ d=\dfrac{v_l}{3}\\\text{Sim:}\ d=\dfrac{5(v_l-15)}{12}[/tex]
Since the left hand sides are equal, we equal the right hand side as well:
[tex]\dfrac{v_l}{3}=\dfrac{5(v_l-15)}{12}[/tex]
Multiply both sides by 12:
[tex]4v_l=5(v_l-15)[/tex]
Expand the right hand side:
[tex]4v_l=5v_l-75 \iff v_l = 75[/tex]
Which implies
[tex]v_s=v_l-15=75-15=60[/tex]