Question

The ratio of lime to sand in mortar is 3:7. How much lime must be mixed with 56 bags of sand to make mortar?

Answer 1

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.