The table shows numbers with the ratio x : y. find the missing number.
x 4 ? 36 42
y 20 120 180 210

This is the concept of functions. Given the table that shows the numbers with ratio x:y. To find the missing number we proceed as follows;
x: 4,_,36,42
y:20,120,180,210
Getting the ratios of each corresponding values we get:
4:20=1:5
36:180=1:5
42:210=1:5
therefore the common ratio is 5. Hence the missing value will be:
120/5=24
The missing number is therefore 24

The correct answer is: b