The Cassette Recorder

This equation is incomplete: 1 2 3 4 5 6 7 8 9 = 100

One way to make it accurate is by adding seven plus and minus signs, for example.

1 + 2 + 3 – 4 + 5 + 6 + 78 + 9 = 100

How can you do it using only 3 plus or minus signs?

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