The classic way to convey the error that exists in a measurement is a formula that shows the obtained measurement (X) as a sum of a true score plus some error. One version of this formula is: X = T + e. With this formula in mind, obtaining more precise measures depends on reducing error.
  Mega is the prefix meaning one million, therefore there are one million (1,000,000) gallons in a mega gallon.
The answer to the question how many gallons are in 50 liters depends on the measure you have. The US liquid gal is 3.785411784 L, the imperial gal is equivalent to 4.54609 L, and the US dry gallon is approximately 4.405 L. When we write 50 liters to US gallons we mean US liquid gallons unless stated differently.