book Maths Puzzle Numerical Ability

4x+9=7 i know the answer is negative 1/2 but i just dont know how my teacher got that, and also z/80= 20/100

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Evaluate. - | -2 | I would like to be able to solve the following for the unknowns a, b, c and d.

a*4 + b*2 + c *9 + d*7 = 21
a*2 + b*5 + c*4 + d*4 = 11.5
a*6 + b*3 + c*5 + d*3 = 15.5
a*8 + b*4