



 
Hi Dilshad, The important fact here is that distance equals time times rate. Suppose that Hassan drives at $v$ kmph and it takes him $t$ hours to travel the 40 km. Then $40 = t \times v.$ His claim is that if he increases his speed to $v + 40$ kmph his time to travel the 40 km will be 20 minutes less. 20 min is $\frac13$ of an hour so at this increased speed he will travel the 40 km in $t  \frac13$ hours. Again using the fact that distance equals time times rate we get \[40 = (t  \frac13) \times (v + 40).\] Expand the right side and use the fact that $40 = t \times v$ to eliminate $v$ from the above equation and solve for $t.$ Finally use $40 = t \times v$ again to find $v.$ Penny  


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