Well lets look at this, he started out with 40,000 KG and ended up with 7600KG which means 40,000-7600 = 32,400 KG was eaten.
Well 32,400 / 18 will give us how much was eaten on average per day = 1800 was eaten per day on average over the course of 18 days.
Well since we know how much was eaten per day (1800) what if we want to find out how much was eaten per day by each bird, well 1800/120 will give us how much each bird ate on average! thus 1800/120 = 15 was consumed per day on average by each bird!
I am a bit confused by part 2 of your question, if one bag covers 2500m sq then you just take the Total # of m sq of the farm / 2500 then you take that number and multiply it by 6.95 but it looks like you didn't provide how many m sq the entire farm takes up.