HOW CAN YOU MEASURE 20 mL OF WATER WITH A 200 mL CONTAINER, A 30 mL CONTAINER AND A 40 mL CONTAINER?

Suppose that you are given a bottle of water and three beakers. One of the beakers holds 30 mL, one holds 40 mL, and the largest of the three beakers holds 200 mL when full. There aren't any markings on any of the beakers. Describe how you could put exactly 20 mL of water in the large beaker without using any other equipment.