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Hello there!

The correct answer is 20.

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We can write a ratio equation 90/100 =18/x to show how many problems he got right on his science test.  

We can cross multiply to simplify our equation to 90x=1800.

Now we divide by 90 on both sides to get x=20.

There were 20 questions on the science test, if you answered 18 questions correct, your test score would be a 90%.