Here is one way to do it (although it took long!)
PQR --> all positive integers, single-digit. This means P,Q,R could be 1,2,3,4,5,6,7,8,9
Is PQR between 100 and 300? This simply translates to whether P is 1 or 2.
Why? Because Q and R are never zero, so when P = 1 or 2, PQR minimum = 111, PQR maximum = 299
Let's look at the statements now.
Statement #1
Q = P+S
R = Q + 0.75S
We want to know what values P will take. So, use these equations --
P = Q - S = R - 0.75S - S = R - 1.75S
For P to be an integer, R - 1.75S must be an integer. So, 1.75S must be an integer. 1.75S = 7S/4. This takes an integer values only when S = 4 or 8
At S = 4, P = R - 7 --> P can be 1 or 2 because R can be 8 or 9 for P to be > 0 and single-digit.
At S = 8, P = R - 14 --> here, R will have to be greater than 14 for P to be a positive integer. That is not possible!
P = 1 or 2. Statement is sufficient.
Statement #2
The approach is same as that for statement 1.
Q = P+T+2 --> P = Q-T-2 = (R-3T/2)-T-2 = R - 5T/2 - 2
Again, to get integer values for P, 5T/2 must be an integer. T = 2,4,6,8.
At T = 2, P = R - 7. Same situation as statement #1. P = 1 or 2.
For T = 4,6,8, R cannot be single-digit.
Sufficient.
Answer = D
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Edit: Just saw that another similar solution is already posted. I hope this helps in any case!