Data Sufficiency

What was John’s average driving speed in miles per hour during a 15-minute interval?
(1) He drove 10 miles during this interval.
(2) His maximum speed was 50 miles per hour and his minimum speed was 35 miles per hour during this interval.
ANS is A
but why not "D"?..

2nd SUM: Is quadrilateral Q a square?
(1) The sides of Q have the same length.
(2) The diagonals of Q have the same length.
ANS: C

3rd Sum: . Is x2 – y2 a positive number?(x2= x square, y2= y square)
(1) x – y is a positive number.
(2) x + y is a positive number
ANS:C

4th Sum: The surface area of a square tabletop was changed so that one of the dimensions was reduced by 1 inch and the other dimension was increased by 2 inches. What was the surface area before these changes were made?
(1) After the changes were made, the surface area was 70 square inches.
(2) There was a 25 percent increase in one of the dimensions.
ANS: D

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!!