Question 1173788
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<pre>

Let x = John' speed, in km/h.


Then the Sue speed is (x+20) km/h.


From the condition, you have this "time" equation


    {{{810/(x+20)}}} = {{{720/x}}},


or, dividing by 90 both sides


    {{{9/(x+20)}}} = {{{8/x}}}.


Cross multiply

    9x = 8(x+20)


and solve

    9x = 8x + 160

    9x - 8x = 160

       x    = 160  km/h for John,

and           160+20 = 180 km/h for Sue.
</pre>

Solved.