January 7 on Average

Karamjit Kaur wrote an article related to an Indonesia AirAsia crash as 2014 came to a close. The unfortunate flight was on its way from Surabaya in Indonesia to Singapore. While we lost we only one Singaporean, we lost several who were closely connected to Singapore. A girl in one of our schools waited for her family who never arrived and a well-loved Biology teacher who taught in one of our junior colleges.

She wrote:

A spokesman for SilkAir, which operates a total of 64 weekly flights to 12 Indonesian cities, including Surabaya, Semarang and Medan, said: "As part of our standard operating procedures, Silkair files for landing and take-off slots from the respective slot coordinators, and then for approval from the relevant aviation authorities, before the start of every flight season. Approvals for our Indonesia routes are all in order." | The Straits Times 7 January 2015

Question | On average, how many flights per day does SilkAir have to each of the 12 Indonesian cities?

Discuss | Why is the value for the average less than one when number of flights have to be whole numbers? Can we say this is the average number of flights from Singapore to Indonesia? Why or why not?

A Little Bit of English and Science |

English Language
Take off is a phrases verb.
In this article, what does to 'take off' mean?
Give another meaning for to 'take off'.
Make two sentences to show the two different meanings of to take off.

What other phrasal verbs for take do you know?

Science
A balloon floats in the air because it is light.
An aircraft with all its content and passengers is heavy.
What make it stay in the air?