Tom was saving $3000 for a trip. he initially deposited $200 in a savings account and then deposited a fixed amount every month in the account for the next 5 months. then he received a check in the mail. the check was three times as much as the total amount he had saved so far. when he deposited the check in his savings account, he had $300 more than he needed. what was the fixed amount he deposited in the savings account during the 5 months?

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