Question 61267
Let population of Boomtown, {{{B=350000+15000y}}} where y is the number of years into the future.

Let population of Ghostville, {{{G=200000-10000y}}}.

We want to know the value of y where {{{B=5G}}}.

So, {{{B=350000+15000y = 5G = 5(200000-10000y)}}}.
Or, {{{350000+15000y = 1000000-50000y}}}.
Subtract 350000 from both sides: {{{15000y = 650000-50000y}}}.
Add 50000y to both sides: {{{65000y = 650000}}} so {{{y=10}}}.

In 10 years Boomtown will have 5 times as many people as Ghostville.

Verify:

In 10 years {{{B=350000+15000(10) = 350000+150000 = 500000}}}.
In 10 years {{{G=200000-10000(10) = 200000-100000 = 100000}}}, which is 1/5 of B.