Question 453181
f - father's age
s - son's age
:
In five years they will be f+5 and s+5
Five years ago they were f-5 and s-5
:
Five years from now it will be
(f+5)=3(s+5)
:
Five years ago it was
(f-5)=7(s-5)
:
These two form system of two equations with two variables, f and s:
(f+5)=3(s+5)
(f-5)=7(s-5)
:
f+5=3s+15
f-5=7s-35, solve for f -> f=7s-30
:
Substitute f in first equation:
7s-30+5=3s+15
7s-25=3s+15, subtract 3s and add 25 to both sides
4s=40
s=10, this is current age for the son
:
f=7s-30
f=7*10-30
f=40, this is current age of the father
:
check
in 5 years, f=40+5=45, s=10+5=15 -> f=3s, ok
5 years ago,f=40-5=35, s=10-5=5  -> f=7s, ok