## College Algebra 7th Edition

$30$ mi/h.
We are given: $d=x+ \frac{x^{2}}{20}$ We know that $d=75$ and must find the speed $x$: $75=x+ \frac{x^{2}}{20}$ $75*20=20x+ x^{2}$ $1500=20x+x^{2}$ $x^{2}+20x-1500=0$ $(x-30)(x+50)=0$ $x-30=0$ or $x+50=0$ $x=30$ or $x=-50$ Since the speed can't be negative, we use the left solution. So the speed must have been $30$ mi/h.