book Maths Puzzle Numerical Ability

Please help me to understand this expression

[7(x-5)-19]-[4(x-6+10]

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book Other Question

A box with a square base no top is to be made from a square piece of tin by cutting out a 3cm square from each corner and folding up the sides. If the box is to hold 48cm3, what size piece of tin should be used. Suppoes the sum of the 151st and the 251st are of an arithmetic seuqence is 10. Find the sum of the first 401 terms in the sequence. Provide 3 examples.