A man traveling 40 miles finds that by traveling one more mi

A man traveling 40 miles finds that by traveling one more mile per hour he would make a journey in 2 hours less time. How many miles per hour did he actually travel?

Solution

st=d
.
st=40
(s+1)(t-2)=40
st+t-2s-2=40
st+t-2s=42
.
t=40/s
s * 40/s + 40/s - 2s = 42
40 + 40/s - 2s = 42
40/s - 2s = 2
40-2s^2=2s
-2s^2-2s+40=0
2s^2+2s-40=0
s^2+s-20=0
(s+5)(s-4)=0
s=4 mph