A car travels 150 km in 3 hours. Speed = 150/3 = 50 km/h.

Speed, Distance, Time Formula — S = D/T with Examples

Question

A car travels 150 km in 3 hours. Find its speed.

This is the foundational question for Class 7 motion chapter — once you own this, the entire chapter falls into place.

Solution — Step by Step

Distance (D) = 150 km, Time (T) = 3 hours. Always list knowns first — it prevents the most common exam mistake of mixing up which formula to use.

\text{Speed} = \frac{\text{Distance}}{\text{Time}}

This is the one formula you must know cold. Every other variation — finding distance, finding time — comes from rearranging this.

S = \frac{150 \text{ km}}{3 \text{ h}} = 50 \text{ km/h}

Division is straightforward here. The answer is 50 km/h.

Speed = 50 km/h. Never leave speed without a unit. In board exams, a number without a unit costs you half the marks for that step.

Why This Works

Speed tells us how much distance is covered per unit of time. When we divide 150 km by 3 hours, we’re asking: “how many kilometres does this car cover in just one hour?” The answer, 50 km/h, means every single hour the car covers 50 km.

The formula $S = D/T$ works because speed is defined as the rate of change of distance. Rate always means “per unit” — so we divide. The same logic gives us kilometres per hour, metres per second, or any other speed unit.

Alternative Method — Finding Distance or Time

The same triangle trick works for all three variations. Write D at the top and S × T at the bottom:

D = S \times T \qquad T = \frac{D}{S} \qquad S = \frac{D}{T}

Example using the same numbers: If we’re told speed is 50 km/h and time is 3 hours, can we get back 150 km?

D = 50 \times 3 = 150 \text{ km} \checkmark

This reverse-check method is excellent for board exams — if you have time left, verify your answer by plugging it back in.

Cover the quantity you want to find with your thumb on the triangle. What’s left tells you the operation. Cover D → you see S × T. Cover S → you see D/T. Cover T → you see D/S.

Unit Conversion — The Sneaky Part

NCERT Class 7 sometimes gives distance in km but asks for speed in m/s, or vice versa. The conversion factors are:

1 \text{ km/h} = \frac{1000 \text{ m}}{3600 \text{ s}} = \frac{5}{18} \text{ m/s}

So 50 km/h in m/s:

50 \times \frac{5}{18} = \frac{250}{18} \approx 13.9 \text{ m/s}

Multiply by 5/18 to go from km/h → m/s. Multiply by 18/5 to go the other way. Write this on your formula sheet.

Common Mistake

Students flip the formula and write Speed = Time ÷ Distance. This gives a completely wrong number — 3/150 = 0.02, which makes no physical sense for a car. Always ask yourself: “does my answer feel reasonable?” A car doing 50 km/h feels right. A car doing 0.02 km/h is barely moving.

The second common error is forgetting units in the final answer. CBSE board marking schemes explicitly award one mark for the correct unit. Writing “50” instead of “50 km/h” is a free mark thrown away.