125 > 50 + 4.25*p
subtract 50 from each side
75 =>4.25p
divide by 4.25
p>17.64
He may invite up to 17 people ( if he doesn't have to pay for himself)
He may invite 16 if he has to pay for himself
Given:
y is directly proportional to x.
when 
To find:
The equation that connects y with x.
Solution:
It is given that, y is directly proportional to x. So,

...(i)
Where, k is the constant of proportionality.
We have,
when
. Substituting these values in (i), we get
Divide both sides by 5.
Substituting
in (i), we get
Therefore, the required equation is
.
Well the opposite of a square is a square root so it would be s=[square root of]60