\(S_{1_{ }}=v_1.t_1\Rightarrow t_1=\frac{S_1}{v_1}=\frac{S}{75}\)

\(S_2=v_2.t_3\Rightarrow t_2=\frac{S_2}{v_2}=\frac{2S}{3.v_2}\)

\(v_{tb}=\frac{S}{t}=\frac{S}{t_1+t_2}=\frac{20km}{h}\)

\(\Leftrightarrow225v_2=60v_2+300\Rightarrow v_2=18,\frac{18km}{h}\)

Ta có: \(v_{tb}=\dfrac{s}{\dfrac{\dfrac{1}{2}s}{v'}+\dfrac{\dfrac{1}{2}s}{v''}}=\dfrac{s}{\dfrac{s}{20}+\dfrac{s}{2v''}}=\dfrac{s}{\dfrac{s\left(2v''+20\right)}{40v''}}=\dfrac{40v''}{2v''+20}=8\)

\(=>v''=\dfrac{20}{3}\left(\dfrac{km}{h}\right)\)

Giải:

Theo bài ra ta có  S 1 = v 1 . t 1 ⇒ t 1 = S 1 v 1 = S 75

S 2 = v 2 . t 2 ⇒ t 2 = S 2 v 2 = 2 S 3 v 2

Theo bài ra ta có  v t b = S t = S t 1 + t 2 = 20 k m / h ⇒ S S 75 + 2 S 3 v 2 = 20 k m / h

⇒ 225 v 2 = 60 v 2 + 3000 ⇒ v 2 = 18 , 182 k m / h