Environmental Chemistry Problem

Assume that algae need carbon, nitrogen and phosphorus in the atomic ratio 106:16:1. What is the limiting nutrient in the lake that contains the following concentrations: total carbon = 20 mg/L, total N = 0.80 mg/L and total P = 0.16 mg/L? If it is known that half
the phosphorus in the lake originates from the use of phosphate detergents, will banning phosphate builders slow down eutrophication?

I have a test in a couple of days and this is one of the practice problems she outlined to do. A step by step answer would help me understand.