Question 769242
Basic concept is r*t=d, r rate, t time, d distance.
Including tailwind help, rate is r+35.
Including headwind hindrance, rate is r-40.


910 mph with tailwind, time t, distance unknown.
{{{r+35=910/t}}}
760 mph with headwind, time t, distance unknown.
{{{r-40=760/t}}}

Those two equations just use the meaning of rate for traveling or motion.  MILES per TIME.  


"in the same time..." The value for t is the same number in both equations.
{{{t(r+35)=910}}}
{{{t=910/(r+35)}}}
and 
{{{t(r-40)=760}}}
{{{t=760/(r-40)}}}


EQUATE the expressions for t:
{{{highlight(910/(r+35)=760/(r-40))}}}
SOLVE FOR r.