The coefficients of the substances give you the ratio of the number of moles.
The 4 before the H2O and the 5 before the HCl tell you that for every 5 moles of HCl created, 4 moles of H2O had to react.
Therefore:

can be used to find the moles of H2O that react.
Since there are no given items, I will give a general answer. Energy....or the lack of it. Examples: Heat, electricity, force (when an item is moving and it impacts something, it heats up...friction is an example of this), etc
$315 is your total cost of all
rent = 45 * numbers of days
d = numbers of days
45d
2 days free
45(d-2) = 315
Divide 45 to both sides
d-2 = 7
Add 2 to both sides
d = 9
So he rented for 9 days.
Hope this helps
The heat of vaporization is when it changes phases from liquid to gas. When it is quite low, it means that it has weak attraction between the liquid particles