The present age of a father is three times that of his elder son. Four years hence, the age of the father will be four times that of his younger son. If the difference between the present ages of the elder and younger child is 6 years, what is the present age of the father?
Select the option that is related to the third number in the same way as the second number is related to the first number and the sixth number is related to the fifth number. 15 : 135 :: 16 : ? :: 18 : 189