`t_(1)=(1//3d)/(12)" " t_(1)=(d)/(36)s`
`t_(2)=(2//3d)/(20)" " t_(2)=(d)/(30)s`
total time `t=t_(1)=(d)/(36)+(d)/(30)=d((1)/(36)+(1)/(30))`
`=d((5+6)/(180))=d((11)/(180))s`
`"Average speed"=("Total distance")/("Total time")=(d)/((11d)/(180))`
`=(180)/(11)=16.4 km h^(-1)`