Differentiate with respect to time as we want the rate at which something is happening not just a length area or volume.
Look at the variables or letters, unless it is a ‘t’, then we will always multiply by the amount d(letter)/dt if not we are treating the letter as if it were the letter t!
Really does ‘r’ look like ‘t’ ?
So what do we do? We use the rules of differentiation ie Trigonometric, Logarithm Chain ... but if it is not a ‘t’ then DON’T FORGET TO WE MUTIPLY BY d(letter)/dt
This is now the General Expression of how the variables are changing towards each other.