book Maths Puzzle Numerical Ability

The integers from 1 to 1000 are printed on a list how many time does the digit 3 appear?

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please help me with this problem i have no idea how to get the answer and i have been working on it for an hour. solve the system of equations.
3x+4y+z=7
2y+z=3
-5x+3y+8z=-31
thank you!!!
I dont understand how you solve equations by using multiplication and division...Can you help me understand it?