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Two-digit natural numbers are formed, with replacement, from the digits 0 through 9. How many numbers are greater than 54? Help with this question. Im not sure why I have been getting the answer wrong. Could you explain? Calculate the pH of 0.0015 M Ca(OH)2 solution. how? Convert x^2 + 2x + y^2 + 2y=0 to polar form? If speciation has never been observed, nor replicated in the laboratory, how can it be considered a scientific "fact"?