I know that to find the average speed of a traveling object, you use the formula:
S=D/T

but how about if you are given the speed and the time and you want to find the distance?

Here is a simple way to think about algebra (which is the maths of solving equations of unknowns).

You simply must do exactly the same to one side as you do to the other.

So in your case, you need to get an equation in the form "D=..."

To do this you need to get rid of the T on the D side of the equation.

You can do this by multiplying both sides of the equation by T.

so you get S*T = D*T/T

Of course, T/T is just 1 so S*T = D

You're allowed to do this because you are multiplying both sides by the same number, this works with addition, division, even exponents. If you have the equation 1+1 = 2, multiplying both sides by 4 isn't going to make it untrue i.e. 4*(1+1) = 4*2 is still true.

I'm just finishing up a master's degree in physics and I still think about algebra in this way.

Easy way to think of it: If you travelled 100 Miles per Hour for 1/2 hour, How far did you go?

You can rearrange the equation S=D/T to give D=ST (multiply both sides by T).

I know that to find the average speed of a traveling object, you use the formula:
S=D/T

but how about if you are given the speed and the time and you want to find the distance?
Very Simple !
just see if s = d/t then d = s x t
put the values of s & t then find d