GMAT Math

Can someone help me on this?? need explainations:

PS 1
How many integeres between 324700 and 458600 have a tens digit 1 and a units digit 3?

PS 2
In n is an integer which of the following CANNOT be a factor of 3n+4?
a)4 b)5 c)6 d)7 e)8

DS 1
If a and b are positive integeres what is the value of a+b?
1) a/b = 5/8
2) The greatest common divisor of a and b is 1


tnks for your help guys

I will try to answer the questions 1 & 2, correct me if I am wrong.

Q1.
looking for how many times 13 appears between 324700 and 458600. 13 will only appear once every 100 number so: subtract 324700 from 458600 , then divide by 100, to how many times 13 will appear

458600 - 324700 = 133900 => 133900/100 = 1339, therefore the number 13 would appear 1339 times.


Q2. Best way is to plug in numbers for n, try {1,2,3,4,5} this should good picture.

3(1) + 4 = 7
3(2) + 4 = 10
3(3) + 4 = 13
3(4) + 4 = 16

From this you can eliminate a, b d, e thus only leaving c as your answer.

here is my thoughts about the third question,
DS 1
If a and b are positive integeres what is the value of a+b?
1) a/b = 5/8
2) The greatest common divisor of a and b is 1

1- first statement ;isnot suffisant at all

2- second statement tells that the GCF of both a and b is 1, it is also insuffisant but we cab conclude that a and b are both products of different prime numbers ( I mean no factor of a is alsoa factor of be neither is it divior of it)

combining 1 and 2, we conclude that 5/8 is not a simplified fraction of a/b, therfore, a is 5 and b must be 8.

So a+b = 13

I hope I am right