Question 1101380
If you only look at part (a),
(Doing for speed of the car; not the lorry)

<pre>
                  SPEED        TIME        DISTANCE
CAR               r            t            280
LORRY            r-20          t+1&1/6      280
</pre>

and using the given time difference,
<pre>
                  SPEED        TIME        DISTANCE
CAR               r            {{{280/r}}}          280
LORRY            r-20          {{{280/(r-20)}}}      280
Difference                     {{{1+1/6}}}
</pre>


Solve for r to get the car's speed:  {{{highlight_green(280/(r-20)-280/r=1&1/6)}}}.

.
.
if no mistakes in algebra steps done on paper, then
r for the Car,

{{{r^2-20r-4800=0}}}


{{{r=(20+sqrt(400+4*4800))/2}}}


{{{r=(20+2*sqrt(4900))/2}}}


{{{r=10+70}}}


{{{highlight(r=80)}}}, for the car;

{{{r-20=highlight(60)}}}, for the lorry