Question

rania runs for 10min. at a uniform speed of 90km/h. at what speed should he run for the next 20min.so that the average speed comes 12km/hr​

Answer 1

Given :-

Time taken to run = 10 min

Uniform speed = 90 km/h

To Find :-

Speed taken for the next 20 min so that the average speed becomes 12 km/h.

Solution :-

We know that,

• t = Time
• s = Speed
• v = Velocity

Using the formula,

$\underline{\boxed{\sf Average \ speed=\dfrac{Speed}{Time \ taken} }}$

Given that,

Velocity (v) = 12 km/h

Velocity (v₁) = 90 km/h

Time (t₁) = 10 min

Time (t₂) = 20 min

Substituting their values,

$\sf v=\dfrac{v(t_1+t_2)-v_1t_1}{t_1+t_2}$

$\sf v=\dfrac{12(10+20)-90 \times 10}{20}$

$\sf v=\dfrac{12(30)-900}{20}$

$\sf v=\dfrac{360-900}{20}$

$\sf v=\dfrac{-540}{20}$

$\sf v=\dfrac{-54}{2}$

$\sf v=-27 \ km/h$

Therefore, the speed is -27 km/h.

Answer 2

Answer:

Given :-

• Raina runs for 10 mins at the uniform speed of 90 km/h and the next 20 mins at the average speed of 12 km/h.

To Find :-

• What is the speed he run for the next 20 minutes, so that the average speed comes 12 km/h.

Formula Used :-

$\boxed{\bold{\large{Average\: Speed\: =\: \dfrac{Total\: Speed}{Time\: Taken}}}}$

Solution :-

Given :

• v = 12 km/h
• v₁ = 90 km/h
• t₁ = 10 mins
• t₂ = 20 mins

According to the question by using the formula we get,

⇒ $\sf v\: =\: \dfrac{{v_1}{t_1} + {v_2}{t_2}}{{t_1} + {t_2}}$

⇒ $\sf {v_2} =\: \dfrac{v(t_1 + t_2) - v_1t_1}{t_2}$

⇒ $\sf {v_2} =\: \dfrac{12(10 + 20) - 90 \times 10}{20}$

⇒ $\sf {v_2} =\: \dfrac{(120 + 240) - 900}{20}$

⇒ $\sf {v_2} =\: \dfrac{360 - 900}{20}$

⇒ $\sf {v_2} =\: \dfrac{- 540}{20}$

⇒ $\sf {v_2} =\: \dfrac{\cancel{- 540}}{\cancel{20}}$

➠ $\sf\red{{v_2} =\: - 27\: km/h}$