Calculate the defects per million opportunities $($DPMO$)$ given the following:

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Blake, owner of Blakester's T$-$shirt Shop, keeps track of customer complaints. For each T$-$shirt sold, there are four possible complaints: T$-$shirt shrinks, poor quality, design wears off, and doesn't fit right. Each week, Blake calculates the rate of T$-$shirt "defects" per total T$-$shirts sold, and then uses this information to determine his company's DPMO. During the past week, his company sold $1500$ T$-$shirts. His company received $20$ customer shrinkage complaints, $14$ poor quality complaints, $12$ design wears off complaints, $8$ doesn't fit right complaints.

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Calculate his firm's DPMO.