Question 915560
A tour bus leaves Sacramento every Friday evening at 5:00 P.M. for a 264-mile trip to Las Vegas. This week, however, the bus leaves at 5:15 P.M. To arrive in Las Vegas on time, the driver drives 8 miles per hour faster than usual. What is the bus's usual speed? 
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let x=bus's usual speed
x+8=bus's faster speed
15 min=1/4 hr
travel time=distance/speed
..
{{{264/x-264/(x+8)=1/4}}}
lcd:x(x+8)
264(x+8)-264x=x(x+8)/4
264x+2112-264x=(x^2+8x)/4
8448=x^2+8x
x^2+8x-8448=0
solve for x by quadratic formula: 
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}}
a=1, b=8, c=-8448
ans: 
x=-96 (reject)
or
x=88
bus's usual speed=88 mph