Let's see if I get the inequalities right

[A_Lynn_Ferguson]

First step in this challenge is to setup the question.
(30 x 5) - 10n >= 100

Step one - brackets.
150 - 10n >= 100

Subtract the 150 total coins from the right side.
-10n >= -50

Divide both sides by -10, and don’t forget to flip the inequality sign.
n <= -50/-10

Final step, division of right side

n <= 5

So if the player gets all 150 coins, they can fill up five times or less before it interferes with their ability to get upgrades.

[garypettie]

Yep, that’s exactly right. Nice work!
If you could mark your solution with spoiler tags, that’d be great. This gives others a chance to work it out too, without accidentally stumbling across your answer first.