The ratio of lime to sand in mortar is 3:7. How much lime must be mixed with 42 bags of sand to make mortar

18

Step-by-step explanation:

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 42 bags of sand, the left side of the number should be 42. Like this:

? : 42

Since we know that 3:7 can be write as 6:14 or 7:21 ... we can even write it as 18:42, it's still 3:7.