i am having trouble with this problem. its been almost an hour and i still only get part of the answer. i dont know what i am doing wrong.

the question is find the inverse of this matrix.

5 5 5

15 25 20

15 30 25

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this is what i ended up with, but i know its wrong because i used the calculator. i'm trying to figure out step by step.

1 -1 .-2

-.6 .4 -.2

1.8 .-6 .4

so far no luck. no matter what i do i still get wrong answers. i tried interchanging rows and all that good stuff but nothing. i must be doing something wrong.