(1) Express 90 km/hr into meters/sec.

Soln. 90 km/hr  = m/sec  =  25 m/sec.

(2) Convert  10 m/sec  into km/hr.

Soln. 10 m/sec  = km/hr  =  36  km/hr.

(3) A man covers a certain distance at 60 km/hr and returns back to the starting point at 40 km/hr. Find his average speed during the whole journey.

Soln. Average speed  =    =   km/hr  =  48  km/hr.

Always construct an equation in the variable which is not required.

i.e.     If  Time is required, then find the equation either in Distance or  in Speed.

    If  Distance is required, then find the equation either in Time or in Speed.

   If  Speed is required, then find the equation either in Distance or in Time.

(4) I walk a certain distance and ride back taking a total time of  40 minutes. I could walk both ways in 65 minutes. How long would it take me to ride both ways ?

Soln. Let the distance be x  km. Then,

( Time taken to walk x km ) + ( Time taken to ride x km )  =  40 min.

⇒  ( Time taken to walk 2x km ) + ( Time taken to ride 2x km )  =  80 min.

But, time taken to walk 2x  km  =  65 min.

∴ Time taken to ride  2x  km  =  ( 80 – 65 ) min  =  15 min.

(5) A distance is covered in 3 hours 48 min. at 5 kmph. How much time will be taken to cover it at 57 kmph ?

Soln. Here, Distances are equal.

i.e.  D₁  =   D₂

∴  S₁T₁  =   S₂T₂

∴  5×  =  57 × T₂

∴  5×   =  57 × T₂

∴  5×    =  57 × T₂

∴  19  =  57 × T₂

∴  T₂  =    hrs.

=   × 60  min.

=   20 min.

∴   Required time  =   20 minutes.

Click here to see more examples