Imagination Ink supplies different colored inks for highlighter pens. Vat 1 has yellow ink, holds 150 gallons and is 4/5 full. Vat 2 has green ink, holds 50 gallons, and is 5/8 full. One gallon of ink will fill 1,200 pens. How many pens can be filled with the existing ink in Vat 1 and Vat 2?

My teacher gave the answer and it is 181,500 pens. I just don't understand how he got that answer. Can someone help me please?

Thanks in advance!