The best way (for me) to go about these questions is to draw a diagram. It may not say much you don't already know but it really helps the visualisation process.
I will go step by step my thought processes when setting up these equations. If you feel you can figure it out from a point, please do continue by yourself and refer back only when you're stuck. Your own thought processes is much better for you than mine.
I draw a simplistic diagram for you here.
BASE ------- x km @ 240 km/h in time t1------> FIRE
BASE <------ x km @ 320 km/h in time t2------ FIRE
Total 35 mins.
Now that you have a diagram look at what you are solving for. You're solving for x, a specific distance.
How can you relate the information you are given with distance?
Let see, we have a time and two speeds. Now we think about what we have learnt and recall that distance = speed x time
There is a trick here. The speed to the fire is different on the return trip, so the time will be different - there will be a time taken to the fire and a different time for the return trip.
Looking at our formula that relates all our information together, we see that time = distance / speed. Luckily we have a total flight time so this becomes a nice little equation: