Both of those numbers are perfect squares, so square root each number separately and you will get your answer.
√(81/100) = √81 / √100 = 9/10 = 0.9
Don't know how far you've progressed as technically the answer is ± 0.9, but you may not have reached the stage of dealing with negative square roots of positive numbers.
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It is -10/9
The square root of 81 is 9 and the square root of 4 is 2!
9 x 9 = 81 and 10 x 10 = 100. So any number 82-99 would have a square root between 9 and 10.
Square root of 81
sqrt(4/81) = sqrt(4)/sqrt(81) = 2/9