To avoid confusion, remember PEMDAS. This is the order of operation.
Parenthesis, Exponents, Multiply, Divide, Add, Subtract
21 ÷ 7 + 20 * 102 * 2 - 3
No parenthesis and no exponents, proceed to multiplication.
20 * 102 * 2 = 4,080
Then, perform division
21÷7 = 3
Then, do addition
3 + 4080 = 4083
Lastly, do subtraction
4,083 - 3 = 4,080
So, 21 ÷ 7 + 20 * 102 * 2 - 3 = 4,080