How to do this problem?

Answer: From ○+○=4

Get 2 = 4,

That is, 0 is half of 4,

Get ○=2.

(that is, ○=4/2=2. )

From □-○= 1

And ○=2

□-2= 1

From 3-2= 1

Get □=3。

(i.e. □=2+ 1=3. )

So χ = 2,

□=3.