SOLUTION: A math test is worth 100 points and has 30 problems. Each problem is worth either 3 points or 4 points. How many 4-point problems are there?

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Question 950221: A math test is worth 100 points and has 30 problems. Each problem is worth either 3 points or 4 points. How many 4-point problems are there?
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

4x+%2B+3%2830-x%29+=+100
4x+%2B+90+-3x+=+100
4x+=+100-90
x+=+10
Answer: there is 10 problems have 4 points
Proof:
=> 4%2A10+%2B+3%2A20
=> 40++%2B+60 points
=>+100 points