If there is a stock generally made up at 50x concentration, how many milliliters of stock will we need to make up 2 liters of agar medium properly?
you dilute it 50 times.
so you want 1 part stock, 49 parts water.
how big is a part? 1 part= 2000ml/50=40ml
so you want one part (40 ml) of stock.