Answer, although I don't know why it's Minus
Distance = Speed x Time
Time = Distance / Speed
Velocity A = Speed A / Time A
Velocity B = Speed B / Time B
You want the two speeds to equal.
S A = V A x T A
S B = V B x T B
V A x T A = V B x (T A + 10 Mins)
90 x T A = 100 (T A + 1/6)
90 T A = 100 T A + 100/6
-10 TA = 100/6
TA = -10/6 (10 / 6 * 60(mins) = 100 Mins
Q.E.D 100 Mins 