12% of what number is 42?

350 is the answer.

If you compare the ratios and find the correlation between the 4 different quantities and you apply it to the number you want, you can get the answer.

12:42

100:x

I put the percents together and the numbers together.

To get from 12 to 100, to multiply by 18.333 repeated and then you multiply 18.33 by 42 to get 350.