Bart recorded the amount of rain that fell in a desert in each month for one year. What was the total amount of rainfall in the desert that year?
(I need the step by step explanation)

Ok so let T=time from when Tim starts
Distance Tim travels= T*90
Distance Dora travels=T*75 + 75  (<as already started)
for him to have caught up His distance must equal hers so:
90T=75T + 75  (now minus 75T)
15T=75  (now divide by 15)
T=5 :)
So five hourse, hope this helped :)