j = johns age
f = franks age
in order to solve this you need a system of equations.
j = f + 4
j + f = 36
we know j = f + 4 so we can just substitute j in the second equation with f = 4 to get a new equation of
f + 4 + f = 36
which simplifies into
2f + 4 = 36.
now we subtract 4 from both sides
2f = 32
and divide 2 to both sides
f = 16
so franks age is 16.
what about johns age?
well we know frank is 16 and johns age is 4 greater than that from the equation j = f + 4
so just plug in
j = 16 + 4
which simplifies to
j = 20
so john is 20 years old and frank is 16 years old.
