assume
t0 - planned arrive time
t1 - time he starts to walk
t2 - time car starts to go
t3 - time he meets car
v1 - his speed
v2 - car speed
x - distance
y = t3-t1 time we wants to solve

time unit is min
t1-t0=-60
t2-t0=-x/v2 because car supposes to arrive on time
t3+(t3-t2)v2/v2=t0+x/v2-20
=>
y-60+(y-60+x/v2)=x/v2-20
=>
y=50

there is another equation you can get
v1(t3-t1)+v2(t3-t2)=x
=>y=60v2/(v1+v2)

but it is useless for the solution so I guess it is just used to distract you

as you said, equation is bad to solve this problem