Because it relates to more than one branch of knowledge. It combines more than one academic disiplines.
Remember....
mass number= atomic number + number of neutrons
If the mass number is 19 and the atomic number is 9, then the number of neutrons is 19-9 which is 10.
Air moves when the molecules are free meaning they aren't close up to each other or trying to fill up space, they are moving freely.
Answer: A tropical year is longer than 12 lunar months and shorter than 13 of them. The arithmetical equation
12x12 + 7x13 = 235
allows it to be seen that a combination of 12 'shorter' (12 months) years and 7 'longer' (13 months) years will be equal to 19=12+7 solar years.
Explanation:
Waters freezing point in Celsius is 0 and 273.15 Kelvin (or 273 rounded to a whole number.
