There are 7 red marbles + 4 blue marbles = 11 marbles.

The probability of choosing a red marble is

7 out of 11

"Out of" means "over", so that's

7 over 11 which is 

That would be the answer but the instructions say

So we change  to a decimal:

We divide the bottom number 11 into 7, but instead of
7 we write it as 7.000, one more decimal place than 2



    .636
11)7.000
   6 6
     40
     33
      70
      66
       4
      
Since the third decimal digit is 6, 
and 6 is "5 or more", we round the
second decimal place 3 up to 4, and
eliminate the third decimal digit 6:

Answer = .64

We like to put a 0 in front, so it
would be better to write the answer
as  0.64

The big 0 is to "protect" the tiny decimal point. :)

Answer = 0.64

Edwin