I bought a cow for $800. I sold it for $1000. I bought it again for $1100. I sold it again for $1300 dollars.
How much did i earn?

$400.

I bought 1 cow for $800.
I sold it for $1000.
Then, earned $1000 – $800 = $200

I bought it again for $1100.
I sold it again for $1300.
Then, earned $1300 – $1100 = $200