How do I solve a equation with two knowns and two unknowns?

Given: Four variables A, B, C and D related by the equation A = (B x C) / (B + D).

Step 1: A and D are known values, derive a equation to determine the relationship between known variables A, D and unknown variables B, C for the next step.

Step 2: Using the equation derived in Step 1 for a different value of A calculate D.

Show both equations and steps taken.

Does this problem have a name or is it used for a pratical purpose?