Can someone me on this question ?

A is 5×10^7 because you are trying to move the decimal to get the number at the beginning to be lower than 10 but not a decimal so you move the decimal 7 times to get 5. Because you moved it 7 times you do ×10^7. The exponent would be negative if you had a small number but for this question it would be positive because it's a big number.

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