Two species, X and Y, live in a symbiotic relationship. That is, neither species can survive on its own and each depends on the other for its survival. Initially, there are 15 of X and 10 of Y. If x = x(t) and y = y(t) are the sizes of the populations at time t months, the growth rates of the two populations are given by the system

x′ = -0.8x + 0.4y
y′ = 0.4x - 0.2y

Determine what happens to these two populations.