Question 721995
usual speed be x

 today's speed = x+4

difference in time = 1/3 hours ( 20 min)

Usual time - today's time = 1/3

{{{144/x - 144/(x+4) = 1/3}}}


multiply equation by 3x(x+4)


{{{144*3(x+4)-144*3*x= x(x+4)}}}


432x+1728-432x=x^2+4x

x^2+4x-1728=0

solve this you will get the usual speed which is 39.62