Question 407195
Let r = rate in the return trip
==> rate in going to grandmother = r + 10
==> {{{200/r  - 200/(r + 10) = 1}}}, using the formula t = d/r.
==> {{{(200r + 2000 - 200r)/(r(r+10)) = 1}}}
==> {{{2000 = r(r+10) = r^2 + 10r}}}
<==> {{{r^2 + 10r  - 2000 = 0}}}
<==> (r + 50)(r - 40) = 0
==> r = 40 miles per hour, the rate in the return trip.