Andrew has these marks: english 82% french 75% history 78% science 80% what mark will andrew need in math if he wants his mean mark to be each percent? i)80% ii)81% iii)82%

Sometimes, the strategy guess and check is the simplest. 82, 75, 78, and 80.
i) Let's try 80. All added together is 395 divided by 5 is 79%, which isn't enough. Now trying 85, the mean would be 80%.
ii) Let's try 90's mean. It's 81%.
iii) First, I'm going to try 95. The percentage mean is 82%
I didn't have to guess for ii) and iii) since I saw a pattern happening.