Consider a primary sedimentation tank (PST) in a water treatment plant with Surface Overflow Rate (SOR) of $40 m^3/m^2/d$. What will be the diameter of the spherical particle having $90$ percent theoretical removal efficiency in this tank ? Assume that settling velocity of the particles in water is described by Stokes’s Law.

Given: Density of water $=1000 kg/m^3$; Density of particle $=2650 kg/m^3$ ; $g=9.81 m/s^2$ ; Kinematic viscosity of water $\left(v=1.10\times {10}^6 m^2/s\right)$

A man covers a certain distance in 10 minutes with the speed of 15km/hr. If he wants to cover the same distance in 7 minutes, then what should be his speed?

Ali travels a distance of $300 \mathrm{m}$ in $2$ minutes and $30$ seconds. What is his speed in $\mathrm{kmph}$?

 A van covers $25 km$ at the rate of $20 kmph$ and again a distance of $80 km$ at the rate of $25 kmph$. Find the average speed of the van (in $kmph$) for the whole journey. 

Two trains are running opposite each other. Speeds of both trains are $112 km/hr$ and $115 km/hr$. Find the relative speed of both trains with respect to each other. 

Find the speed of the bike in m/sec which covers a distance of $4\times {10}^4$ m in ${10}^{-2}$ seconds.

Gopal covers $100 \mathrm{km}$. He travels at a speed of $60 \mathrm{kmph}$ for the first $40 \mathrm{km}$ and the rest of the distance at $40 \mathrm{kmph}$. What is his average speed in $kmph$?

1. $44.20$
2. $45.20$
3. $46.20$
4. $47.20$