book Maths Puzzle Numerical Ability

can someone help with solving this problem?
cubed root(x^2) = 4

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book Other Question

[(1+a/b)(1-a/b)]+(a/b-b/a) The letters a, b, c, and d will be used to form passwords for the schools computer system. How many 4 letter passwords maybe formed if the letters can be used more than once in any passwrd?