Question 979903


Let  b  be the Bill's age; 

        p  be the Phil's age;

        j  be the Jenny's age;

and  d  be the Dad's age.


Then you have 3 equations

b = 2p,

j = b - 2,

d = 2*(b + j + p).


In nine years Bill's age will be (b+9); Jenny's age will be (j+9); Phill's age will be (p+9); and Dad's age will be (d+9). 


Thus you have the 4-th equation 


d+9 = (b+9) + (j+9) + (p+9).


From the last equation you have


d = b + j + p + 18.


Compare it with the third equation, and you easily get 


b + j + p = 18. 


So, you have now the system of three equations 


{{{system(b = 2p,
j = b-2,
b + j + p = 18)}}}.


Substitute  b  from the first equation into the second and third, and you will eliminate  b  and reduce the system to the one of two equations in two unknowns:


{{{system(j = 2p-2,
3p + j = 18)}}}. 


Next,  in the obtained system,  substitute  j  from the first equation to the second one.  You will get


3p + 2p -2 = 18,   or   5p = 20.


Hence,  p = {{{20/5}}} = 4.


And so on . . . 


Good luck!