Use PEMDAS
P Parentheses first
E Exponents (ie Powers and Square Roots, etc.)
MD Multiplication and Division (left-to-right)
AS Addition and Subtraction (left-to-right)

Answer:
x=16
Step-by-step explanation:
Answer:
840
Step-by-step explanation:
120 is 40% of 300, and 840 is 40% of 2100 is 840.
*edit
Answer:
4/5
Step-by-step explanation:
you'd do 4 divided by 2 which is 2, then you'd do 10 divided by 2 which is 5. so you get 4/5.