If a hummingbird were to get its food from a feeder, then a 16 ounce feeder could feed 80 birds. How many birds can feed with a 12 ounce feeder?

16 ounce feeder could feed 80 birds.

12 ounce feeder could feed how much?

Calculate it like this:

(12*80) / 16 = 60

Therefore, a 12 ounce feeder can feed 60 birds.

Its very simple. Write down the statements with the same quantities below each other with the? mark.

Later cross multiply the bottom left quantity with the top right quantity and divide by the top left quantity.

This will work only when the quantities increase or decrease proportionally. It will not work in other cases.

Simple example,

Cost of 1 pen is 2$

Cost of 2 pens is?

(2*2) / 1 = 4$

Cost of 2 pens is 4$.

This will not work when one quantity increase causes the other quantity to decrease. But in this scenario it will work like magic, its the best shortcut I know.