Soln to 1st question:

Let the no. of male student is x and female student be y, thus total no of student is = x+y

Ques gives information that 72% of X and 80 % of Y applied for the college and asks what is x/x+y

Statement 1:
indicates that x+y =840, therefore x/x+y = x/840 we cannot answer the question as X is unknown.

Statement 2:
Indicates that: 75% of x+y applied for the college thus-

72% of X and 80 % of Y = 75% of x+y

72/100 * X + 80/100*y = 75/100 (x+y)

from this we can get x in terms of y and thus we can calculate x/x+y

So statement 2 is enough to solve the problem.

Solution to second prob:

Question ask are x and y both +ve

Statement 1:
2x -2y = 1 or x-y =1/2
Hence x and y can both be positive and negative
x = 1 and y = 1/2
x = 1/2 and y =-1

not sufficient

Statement 2:
x/y> 1 also implies x>y, please note here x or y cannot be zero as their ratio is greater than 1
x can be +ve as well as -ve, so not sufficient

By combining both the statement:
x-y =1/2 and x>y and x is non 0

x and y must be +ve to satisfy x-y=1/2 equation.

So both statement togther can answer the question.

Answer to 3rd question.
https://www.beatthegmat.com/gmat-prep-tr ... 12976.html