Could someone please get me started on this question, I have no idea where to begin. Thanks heaps.

If two resistors with resistances R1 and R2 are connected in parallel, then the total resistance R, measured in ohms is given by:

1/R = 1/R1 + 1/R2

If R1 and R2 are increasing at rates of 0.9/s and 0.5/s respectively, how fast is R changing when R1 = 50 and R2 = 115?