12,5m/s=45km/h

Quãng đường còn lại : \(75.\left(1-\dfrac{1}{3}\right)=50\left(km\right)\)

\(\dfrac{1}{3}\) quãng đường : \(75\cdot\dfrac{1}{3}=25\left(km\right)\)

a, \(t_1=\dfrac{s_1}{v_1}=\dfrac{25}{50}=0,5\left(h\right)\)

\(t_2=\dfrac{s_2}{v_2}=\dfrac{50}{45}\approx1,1\left(h\right)\)

b,\(v_{tb}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{25+50}{50+45}\approx0,79\left(kmh\right)\)

11d

sang

didn't have