Q72 / 90 · JEE Main 2024 (06 Apr Shift 1) · Physics

A train starting from rest first accelerates uniformly up to a speed of $80 \mathrm{~km} / \mathrm{h}$ for time $t$, then it moves with a constant speed for time $3 t$. The average speed of the train for this duration of journey will be (in $\mathrm{km} / \mathrm{h}$ ) :

  • A
    40
  • B
    80
  • C
    30
  • Correct answer
    D
    70

Solution

$\begin{aligned} \text { Average speed } & =\frac{\text { total distance }}{\text { time taken }} \\ & =\frac{\frac{80 \times \mathrm{t}}{2}+80 \times 3 \mathrm{t}}{4 \mathrm{t}}=70 \mathrm{~km} / \mathrm{hr} .\end{aligned}$

Hence, the correct answer is option: 'D'.

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