For distance, speed and time questions, we use a formula like this:
D is for distance, S is for speed, T is for time.
D = S.T, S = D/T, T = D/S
For motion problems, for example: if a current is 5 km/h, the upstream will be x-5 and the downstream will be x+5, x being the speed without current.
For proportion problems, for example if theres a mass of 12 kg of a substance and if we want another substance to be %40 of the mixture, we use x/x+12 = 40/100.