F.5 MATHS

The combine speed of A and B is (t + 400) + (t + 300) = 2t + 700 m/min

So, 7200 <= t(2t + 700) <= 8000

For t(2t + 700) - 7200 >= 0

2t^2 + 700t - 7200 >= 0

t^2 + 350t - 3600 >= 0

(t - 10)(t + 360) >= 0

t <= -360 or t >= 10

For t(2t + 700) - 7200 <= 0

2t^2 + 700t - 8000 >= 0

t^2 + 350t - 4000 >= 0

-361 <= t <= 11.078

So, the range of t is 10 <= t <= 11.078