Question 1196999
A D mile, T hour plane trip was flown at two speeds. For the first part of the trip, 
the average speed was L mph. For the remainder of the trip, the average speed was G mph. 
How long (in hours) did the plane fly at each speed?


<pre>
               SPEEDS         TIMES       DISTANCES

LOWER SPEED       L             T-x        L(T-x)

GREATER SPD.      G              x         Gx

TOTAL                            T           D
</pre>
{{{highlight_green(L(T-x)+Gx=D)}}}


{{{LT-Lx+Gx=D}}}

{{{(G-L)x+LT=D}}}

{{{(G-L)x=D-LT}}}

{{{highlight_green(x=(D-LT)/(G-L))}}}


Your example gives 
D=810
L=111
G=122
T=7


Substitute and evaluate the unknown time, x.
Use it to evaluate T-x.