What is the number of liters in 90 gallons

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He mass of a single atom of carbon can be found by dividing the atomic mass (12.01 g) by 6.022 x 10^23. which is the mass of a single carbon atom, correctly written in scientific notation with the correct number of significant figures?

Gabrielle's age is two times mikhail's age. the sum of their ages is 84. what is mikhail's age?