1st SUM:

1) drove 10 miles for 15 mins that gives, the avg speed as 40 miles/hr.
So, SUFF
2) max speed of 50 miles/hr and min speed of 35 miles/hr.
here the distance covered will always change hence miles/hr covered will be differennt everytime, take the following conds
a) 10 mins with 50miles/hr 5 mins with 35miles/hr,
b) 10 mins with 35miles/hr and 5 mins with 50miles/hr and so on
everytime the ans will be diff, so INSUFF

Ans A

2nd SUM:

Attached

3rd SUM:

x2 - y2 = (x+y)(x-y)

1)
a) let x = 2 and y = -7
(x-y) = 2-(-7) = 9; positive but
x2 - y2 will be negative.
b) let x= 7 and y = 2
(x-y) = 7-2 = 5; positive and
x2 - y2 will also be positive. so INSUFF

2)
a)let x= 2 and y = 3
(x+y) = 3 + 2 = 5; positive and
x2 - y2 will be positive
b) let x = -2 and y = 3
(x+y) = -2 + 3 = 1; positive but
x2 - y2 will be negative. so INSUFF

both statements together will do so,
Ans C.

4th SUM:
1) let x be the side of the square.
we can form the eqn as
(x-1)(x+2)=70
x2 + 2x - x -2 = 70
x2 +x -72 = 0
x2 +9x - 8x -72 = 0
x(x+9) -8(x+9) = 0
(x+9)(x-8) = 0
x can not be negative so we will discard x = -9;
x = 8; so area of the original square is 64; So SUFF

2) one of the side of the square is increased by 25%
let x be the side of the square.
so 1.25x will be one of the dimensions and other will be x
so area of the changed tabletop will be x * 1.25x
and the are of the square will be x2;
So for every distince value of x both the area's can be calculated, So SUFF

Ans D.

dear thread starter,
use relevent title to the thread that would attract more (desired) visitors to read your thread.
& that will eventually save you from posting a duplicate threads

seriously, it's not encouraged.

thank you so much!!