Question

6. A car is moving with a speed of 90
km / hour its speed in metre / second
is​

Answer 1

$Easy \: \: way:$

First, since there are 60 min. in an hr, divide 90 km/hr by 60 to get km/min. To make the division easy, drop the zero off both 90 & 60 (essentially dividing both by 10). Now you have 9/6 which is 1.5 km/min.

Next: 20 sec. is 1/3 of a minute so in 1/3 min you travel 1/3 of 1.5, or 0.5 km in 20 seconds.

Conveniently these values lended themselves well to easy arithmetic. Obviously that is not always the case. However, if you just want an approximate answer, you can do the math in your head by rounding the beginning numbers to convenient numbers that are close to their value.