Hi guys, I'm reading this problem which I find quite confusing, here it is:

"We are in 2100 and we can travel universe, the α1λ3 course links the Earth to Saturn and spaceships are obliged to follow it taking several years to get through it. In order to watch the spaceships in their course, the first day of each month of each year are calculated the covered light year distance from the earth, if the spaceship takes off from the earth, and the light year distance left to get to the earth if the spaceship takes off from Saturn. Suppose that the two spaceships travel at different velocity and have taken off at the same time, the first from the earth and the second from Saturn on the 1st of January of the same year ( N ), and have got to their destination the 1st of January of the year M1 the former, and the 1st of January of the year M2 the latter. (M1, M2 > N).

DA and DB are two arrays that contain the distances covered by the two spaceships, given the three years (N, M1, M2), find in which year and in which month the two spaceships have crossed each other.

Example:

for a given year N the two arrays are:

DA = [0, 0.5 , 0.7 , 1.2, 1.5, 2]

DB = [2, 1.7, 1.3, 0.9, 0.4, 0.24, 0]

and the two spaceships crossed in April (the 4th month) of the year N

Guys this is not an homework, I'm not asking anyone to solve it, I just need to understand the mathematical concept behind it, i made some graphs but I couldn't figure out its logic, any tip would be appreciate, thanks anyway