exam Maths Puzzle Numerical Ability

simplify

(a^-1 + b^-2)^-1

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exam Other Question

((-6.9+7.7)/-2 )+1

OK this one I am totally LOST. I do not no what any of this means what is the (( and the /-2 mean?? I cant even find this in my textbook. Can someone please explain to me how this problem is actually work out? Thank you much. Find the value of x: log x 25 over 9 = 2