Question 172631
A father, being asked his age and that of his son, said,
 "If you add 4 to my age and divide the sum by 4, you will have my son's age.
 But 6 years ago I was 7 1/2 times as old as my son."
 find their ages.
:
Let f = father's age now; s = son's age now
:
"If you add 4 to my age and divide the sum by 4, you will have my son's age."
{{{((f+4))/4}}} = s
Multiply equation by 4
f + 4 = 4s
f = 4s - 4
:
"But 6 years ago I was 7 1/2 times as old as my son."
f - 6 = 7.5(s - 6)
f - 6 = 7.5s - 45
f = 7.5s - 45 + 6
f = 7.5s - 39
Substitute (4s-4) for f in the above equation
4s - 4 = 7.5s - 39
-4 + 39 = 7.5s - 4s
35 = 3.5s
s = {{{35/3.5}}}
s = 10 yrs old is the son
then
f = 4(10) - 4
f = 36 yrs old is the father:
:
:
Check solution in the statement:
"If you add 4 to my age and divide the sum by 4, you will have my son's age."
{{{((36+4))/4}}} = 10; confirms our solution 



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