Question 824369
"the car consumes 6lit/hr when travelling 75km/hr."
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Let r= speed and E = fuel consumption rate.
{{{rk=E}}};
{{{k=E/r}}}
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Putting the given value and computing,
{{{k=6/75=2/25}}}


Your question description is unfortunately confusing.  My attempt to help is just this:
90 km/hr means, {{{90(2/25)=E}}} in liters per hour.
{{{E=7&1/5}}} liters per hour.
You say, 20 L were used for the roundtrip, so {{{20*liters(1*hour/(7.2*liters))=2.78}}} hours of roundtrip travel time.


I have no idea what the 6 days and the Sunday information means.  You need to clean or clarify your problem description and maybe recheck what question you want to ask.