Try using PEMDAS,
Parentheses
Exponents 
Multiplication
Division
Addition
Subtraction
So just do the problem in that order.
        
             
        
        
        
Answer: the answer to your question is 
Recall that when you multiply a decimal by a power of ten (10, 100, 1,000, etc), the placement of the decimal point in the product will move to the right according to the number of zeros in the power of ten. For instance, 4.12 · 10 = 41.2.
hope this helps for what you needed have a nice day! :)
 
        
             
        
        
        
The prime factorization of 640 can be written as 27 × 51 where 2, 5 are prime.