book Maths Puzzle Numerical Ability

I am not sure how to solve this problem, please help me!

2/5 = /10

2 over 5 equals some number over10

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book Other Question

x is not greater than or equal to 0. then Root of x root of x root of x is? The length of an airport is 1 mile more than twice its width. The area is 21 square miles. Identify the length and width of the airport.
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