Mathematical logic problem

Everyone is divided into three groups, with 50 people in each group, all of whom are ABCs.

The truck takes group A first, BC goes, and the time is T 1 hour. So go to time a. KM, BC, forward? Km. At this time, A gets off and walks, and the truck goes back to pick up B ..

When I met a truck in BC, T2 hours passed. So a, b and c all go forward again? Kilometers

Then B gets on the bus and goes to T3 hours, which means B goes? KM, a, c forward? Kilometers

B gets off the bus and the truck goes back to pick up C. T4 hours will pass. ABC forward? kilometre (km)

Then ABC reaches the finish line at the same time. Time T5, C goes forward? KM, AB forward? kilometre (km)

That is, the equation a: 70 * t1+10 * (T2+T3+T4+t5) = 90.

Equation b: 70 * T3+(t1+T2+T4+t5) = 90.

Equation c: 70 * t5+(t1+T2+T3+T4) = 90.

Truck equation: 70*(T 1-T2+T3-T4+T5)=90.

Time equation: T 1+T2+T3+T4+T5=4.

So there is a solution!

I made a rough calculation, T 1=T3=T5=37/42.

The rest will do.

Come on, huh?