Initial is 200
Amount deposited each month = d
Current equation is '200 + 5d'
We know he gets a cheque worth 3x what he has so far, so 3*(200+5d) = 600+15d
He adds this to what he has, do 200 + 5d + 600 +15d, or simply
800 + 20d
We know that this amount is 300 more than the amount be needs (3000), so 800 + 20d = 3000 + 300, or
800 + 20d = 3300
Subtract 800 from both sides, to get
20d = 2500
Divide both sides by 20 to isolate d, so you end up with
d = 125
He deposits $125 per month