What am I doing wrong with these simple equations for math?

[zEkii]

Advertisement - Members don't see this ad

http://i1252.photobucket.com/albums/hh572/azizmay94/IMG_4123.jpg

The question asked "A father’s age is twice the sum of the ages of his four sons. In 30 years, the total of the sons’ ages will be twice the age of their father. How old is the father now? " . My work is in the link. My answer is wrong, I would like to know why. I am confused

 

[Lnguyen7]

Let the age of the 4 sons be a, b, c and d and of the father be e

A father’s age is twice the sum of the ages of his four sons
2(a + b + c + d) = e

In 30 years, the total of the sons’ ages will be twice the age of their father
(a + b + c + d + 120) = 2(e + 30)

Solving for e = 40.
Is it correct?

I think you missed to take into account the age for all of them after 30 years.

 

[zEkii]

Ahh that makes sense, I just assumed that they were all the same age and so I just made a+b+c+d a constant. Thank you!