If you could make one thing in the world free to everyone, what would you choose and why? How would this change the world?

Education. All the way from primary school to university

Explanation:

Free education is the best means to achieve equality and fight poverty that I can think of. There are still issues with it, though. In absolute poverty, even if school is free kids might still be needed for work and rich people will still have an advantage because they can pay for tuition etc.

the boy's acceleration toward the girl is 2.08 m/s^2.

explanation:

according the newton's third law, the force causing the girl accelerate towards the boy will be me with equal counter-force causing the boy to accelerate towards the girl. these forces will be of equal magnitude and allows us to determine the boy's acceleration:

the boy's acceleration toward the girl is 2.08 m/s^2.