Answer :
The ratio of the sand is 3:7 and it can be write as 6:14 or 7:21 ... They means the same thing.
Now we know that we have 56 bags of sand, the left side of the number should be 56. Like this:
? : 56
Since we know that 3:7 can be write as 6:14 or 7:21 ... we can even write it as 24:56, it's still 3:7.
