So, in this problem

4 + 10 / 2   We do the division first!

4 + 10 / 2 = 4 + 5 = 9

 

What do we do first here?

20 - 3 x 4

 

PEMDAS

Multiplication gets done before subtraction!
 

20 - 3 x 4 = 20 - 12 = 8