Question 903622
Hi there, the following problem really has me stumped.  Could you help me solve it, please?

A jet plane, flying 80mph faster than a propeller plane, travels 3960 miles in 2 hours less time than the propeller plane takes to fly the same distance.  How fast does each plane fly? 

I ended up solving for "r," in the d=rt formula, and got propeller plane flies at 1956 mph and jet plane 2036 mph. Both answers, however, are incorrect.

Answers cannot have decimals, must either be rounded or fraction form (it's an online homework system).

Thank you!
<pre>
Let speed of propeller plane be S
Then speed of jet = S + 80
Therefore, {{{3960/(S + 80) = 3960/S - 2}}}
3,960S = 3,960(S + 80) – 2(S)(S + 80) -------- Multiplying by LCD, S(S + 80)
{{{3960S = 3960S + 316800 - 2S^2 - 160S}}}
{{{2S^2 + 160S + 3960S - 3960S - 316800 = 0}}}
{{{2S^2 + 160S - 316800 = 0}}}
{{{2(S^2 + 80S - 158400) = 2(0)}}}
{{{S^2 + 80S - 158400 = 0}}}
(S – 360)(S + 440) = 0
S, or speed of propeller plane = {{{highlight_green(360)}}} mph
Speed of jet = 360 + 80, or {{{highlight_green(440)}}} mph