Question 522747
I would do it this way
:
John is 3 times younger than his dad's age.
d = 3j
but they want John's age as a fraction, divide both sides by 3
j = {{{1/3}}}d
:
The sum of their ages is 79.
d + j = 79
:
replace j with {{{1/3}}}d
1d + {{{1/3}}}d = 79
:
1 1/3 = 4/3, so write it
{{{4/3}}}d = 79
: 
multiply both sides by 3, to get rid of the denominators
4d = 3(79)
4d = 237
d = 237/4
d = 59.25 is Dad's age
then
j = {{{1/3}}}59.25
j = 19.75 or 19{{{3/4}}} yr is John's age
:
:
Check this by finding the sum of their ages
59.25
19.75
-----
79.00