book Maths Puzzle Numerical Ability

I need this answer quickly please:
You have 2 hourglasses-a 4 min. hourglass and a 7 min. hourglass. You want to measure 9 min. How do you do it?
Thanks

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i need help on this Problem:work must be shown.

2=x/5-2 Here is the question:
2v(3v2-1) - (6-8v3+14v)+3

here is my attempt:
6v3-2v - 6+6v2+3
4v2-3=6v2

What is the rule with exponents ?

~Tabitha