You are correct that two equations are needed and the problem is not simple IF the calculation was to go 600 steps. However, the OP is only trying to get the calculation correct for step "one." Since the initial y value is given, you don't need dy/dt to solve step one for dx/dt. You just plug...
I think Modulus85's problem is simpler than everyone is making it out. Modulus: It appears you are confusing dx/dt (the CHANGE) with the resulting population total. Your calculation is correct, and it's as simple as that, however, the equation is for the CHANGE in population, not the...