exam Maths Puzzle Numerical Ability

hello please help me i dont understand this proof im confused thanks alot i appreciate it

u need the book to solve it

its Problem number 20 on page 138

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exam Other Question

what is the answer to A grocer mixes two kinds or nuts. One kind costs $5.00/kg and the other $5.80/kg. How manykilogramsof each type are neededto make 40kg of a blend worth $5.50. 5 divided by x2 - 16 minus 3 divided by x2 - x - 12