The argument states that, now, the profit is 20% greater than what it was 5 years ago. It could be possible that the profit was very high 7 years ago before the change in the ownership, the sixth year it hit a rock bottom and starting 5th year, the profit is steadily increasing.
Since, the conclusion is "Employee ownership and a good advertising agency have combined to make Mainline more profitable", we cannot say this for certain without knowing the current profits.
What is OA?
To find what you seek in the road of life, the best proverb of all is that which says:
"Leave no stone unturned."
-Edward Bulwer Lytton