Differential equation

Do I sub in my t and N to find c now or should I rearrange to get N=… and then sub in to find c?

Do I sub in my t and N to find c now or should I rearrange to get N=… and then sub in to find c?

In this case it's probably easier to find the value of c earlier rather than later. You can do that by plugging in the values of N and t given as the initial condition. Then you can rearrange the equation to make N the subject.

Thanks, what do I look for to decide? Because I always try to find it as soon as possible because I feel like I’ll lose values but then the mark scheme doesn’t do that.

There's no hard and fast rule (or at least not one that I'm aware of). The best I can suggest is that you pick a few questions and try finding the value of c at various different points in each of those questions, to develop a feel for what works best for you.

Just one thing I'll add is that if you find yourself with an equation something like ln(y) = kt + c or y = e^(kt + c), it can be better to move on to y = Ae^(kt) then use the initial condition to find A rather than c. But try it for yourself and see what works best for you.