John is four years older than frank the sum of their ages is 36. write an equation

4+_=36 that's an easy equation you can use

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.