Question 1139969

Mary makes monthly visits to her parents in Malacca, a distance of 240 km from Singapore.She finds that if she increases the speed by 10 km/hr , she could save a total of 20 minutes.
Find the original speed.
<pre>Actual time taken, with S being the speed: {{{240/S}}}
Hypothetical time, with S + 10 being the hypothetical speed: {{{240/(S + 10)}}}
Equation you need: {{{matrix(1,7, 240/S - 240/(S + 10), "=", 20/60, "=====>", 240/S - 240/(S + 10), "=", 1/3)}}} ------ Converting 20 minutes to an hour(s)
3(240)(S + 10) - 3(240S) = S(S + 10) ------- Multiplying by LCD, 3S(S + 10)
{{{matrix(1,3, 720S + "7,200" - 720S, "=", S^2 + 10S)}}}
{{{matrix(1,3, S^2 + 10S - "7,200", "=", 0)}}}
Factors of the above TRINOMIAL: + 90 and - 80
Original speed or {{{highlight_green(matrix(1,4, S, "=", 80, "km/h"))}}}       OR   S = - 90 km/h (ignore)