book Maths Puzzle Numerical Ability

find the value of each variable.
3x(80-x)

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book Other Question

7x-4y=-3
2x+5y=-7 One-fifth of a number added to the number itself gives 42. Find the number. If I set this up as 1/5x + x = 42 and mutplied each side by 5, but this does not do it. I end up with 2x = 210. I know the answer is 35. How do I get to 6x = 210?