Move the decimal so there in one non-zero digit to the left of the decimal point. The number of times the decimal point is moved will be the exponent to 10. If the decimal is being moved the right then the exponent will be negative. if the decimal is being moved to the left the exponent will be positive. Therefore, since 0.0078 is converted to 7.8 the decimal point was moved the right three times making the exponent -3.
