Hello,
Let's [tex] v_{1}\mbox{ the speed of David },\ t_{1} \mbox{ the time of David}\\
v_{2}\mbox{ the speed of Sione },\ t_{2} \mbox{ the time of Sione}\\\\
v_{2}=v_{1}+4\ (1)\\
t_{2}=t_{1}- \frac{1}{2}\ (2) \\
150=v_{1}*t_{1} \ (3)\\
150=v_{2}-t_{2} \ (4)\\
(3) \ and \ (4)==\textgreater\ 150=v_{1}*t_{1}=(v_{1}+4)*(t_{1}- \frac{1}{2} )\\
==\textgreater\ 8*t_1-v_1=4==\textgreater\ t_1= \frac{v_1+4}{8}\ (5)\\
(3)\ and\ (5)==\textgreater\ v_1*\frac{v_1+4}{8}=150\\
==\textgreater\ v_1^2+4v_1-1200=0\\
[/tex]
[tex]\Delta=16-4*(-1200)=4816\\
v_1= \frac{-4\pm 4\sqrt{x} 301}{2} \\
v_1=-2+ \sqrt{301} =32.6987031... (km/h) \ since \ (v_1\ \textgreater \ 0)\\
[/tex]
