Find the average speed of the aircraft if it is known that the first third of the way it flew at a speed of 700 km / h

Find the average speed of the aircraft if it is known that the first third of the way it flew at a speed of 700 km / h, the second third of the way at a speed of 500 km / h, and the last part of the way at a speed twice the average speed in the first two sections of the route.

Vc=S/t; t=t1+t2+t3; t1=S/(3*v1); t2=S/(3*v2); Vc1=((2/3)*S)/(S/(3*v1)+S/(3*v2)); Vc1=(2*v1*v2)/(v1+v2); t3=S/(3*Vc1);
Vc=S/(S/(3*v1)+S/(3*v2)+S/(3*Vc1)); Vc=(3*v1*v2*Vc1)/(v2*Vc1+v1*Vc1+v1*v2); Vc1=(2*700*500)/1200=3500/6;
Vc=(3/(1/700+1/500+3/3500)=(3*3500)/(5+7+3)=700