Question 223883
: John is twice as old as his son.
J = 2S 
:
In 42 years, the ratio of their ages will be 4:3.
:
{{{(J+42)/(S+42)}}} =  {{{4/3}}}
:
 what is the sons current age
:
Replace J with 2S in the above equation
{{{(2S+42)/(S+42)}}} =  {{{4/3}}}
Cross mutiply, find S
3(2S + 42) = 4(S + 42)
:
6s + 126 = 4S + 168
:
6S - 4S = 168 - 126
:
2S = 42
s = {{{42/2}}}
s = 21 is Son's age:
:
:
See if that is true
{{{(42+42)/(21+42)}}} =  {{{84/(63)}}} =  {{{4/3}}}