What is 20×40+3×9÷3+4

The answer to that problem is 813. When you are given a problem like this, you can answer it by using the MDAS technique. MDAS stands for multiplication, division, addition, and subtraction. Since you are following the MDAS technique, you are asked to answer the equation based on MDAS order. To put this simply, look at the example below:

     20x40+3x9/3+4
     800+3x9/3+4
     800+27/3+4
     800+9+4
     809+4
     813  

You can clearly see the MDAS order by following the bold text.    

Multiplication and division must be done before addition and subtraction. Therefore:

(20×40)+(3×9÷3)+4
=800+9+4
=813