J = 150 + 2y
Setting those equal, you get 200 + y = 150 + 2y, and solving for y you get that y = 50. Plugging back in, you see that E = J = $250, which is what we originally got.
The significance of the y = 50 is that Emily got $50 from the $1000 of advertising revenue, because her share was 5%. Jacob got 10% of the $1000, which would be $100.
There's still a third way to do this problem: Emily gets HALF as much revenue as Jacob. So, let z = the amount of extra revenue JACOB would have earned from advertising. Then you get:
E = 200 + z/2
J = 150 + z
Setting E = J, you get 200 + z/2 = 150 + z. Solving that for z, you get z = 100. In this problem, z represents the 10% share that Jacob got from the $1000 of advertising.