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k2 wrote:I took the GMAT prep exam and got 700 but I did get stumped by some easy questions and from looking at the answers still cant figure out how they got there so any help would be appreciated:

what is the greatest prime factor of 4^17- 2^28?

a)2
b)3
c)5
d)7
e)11

Five pieces of wood have an average (arithmetic mean) of length of 124 cm and a median length of 140 cm. what is the max possible length, in com, of the shortest piece of wood?

a) 90
b) 100
c) 110
d) 130
e) 140

1) 4^17 - 2^28 = (2^2)^17 -2^28
=2^34 - 2^28
=2^28.2^6 - 2^28
=2^28(2^6-1)
=2^28(63)

Greatest prime factor of 2^28 is 2, and of 63 is 7.

So D

2) Can be done by substituting answer choices. Start with C

110+ _ + 140 + _ + _ = 124.5 = 620

3 pieces must add up to 620 - 250 = 370

2nd piece can be between 110 and 140 (both inclusive).

If 2nd piece is 110, then one of the other two pieces will be less than 140. So 1st piece must be less 110

Choose B

100 + _ + 140 + _ + _ = 620

or 3 pieces add up to 380. If 2nd piece is 100, the other two pieces can be 140.

So option B.

Am I correct? This was very tricky and Im still not very confident with my answer

the second question can be answered as follows


Total length = 124 * 5 = 620

Median = 140
hence 2 values below 140 and 2 above 140
Let us take the Minimum values for the values above 140 as we need Maximum value below 140
so they can be = 141,142

so remaining is 197.the maximum value can be 98.5

hence the possible Maximum value that is available is 90.

Please correct me if it is wrong

sorry there was a mistake the values can be equal to 140, so remaining is 200 and the Maximum value is 100


So I go with option B