The answer to your first question is (e) i.e. neither of the statements is sufficient to answer this question.
(I) rs = 1, this doesnt tell us about the value of t, so we are not sure whether rst=1. So this is INSUFF.
(II) sr = 1, this doesnt tell us about the value of t, so we can not say that rst = 1. So this is also INSUFF.
Even if we combine both of the above arguments, we can not be sure if rst = 1. So the answer is (e).

As far as the second question is concerned, (b) is the correct answer. Let us analyze both the arguments :
(I) 90<r<1000, it tells us about the value of only r. Let us assume a case when r=92 and s=3. Here, r/s is not a terminating decimal. SO (I) is insufficient.
(II) s = 4, when you divide any number by 4, it always returns a terminating number. let us assume r = 3. Here r/s would be 3/4 = 0.75. another ex, r=6, r/s=6/4=1.5, r=9, r/s=9/4=2.25. Thus, any integer divided by 4 always gives a terminating decimal. Hence (II) is SUFF and answer is (b).

tar.goyal wrote: (II) s = 4, when you divide any number by 4, it always returns a terminating number. let us assume r = 3. Here r/s would be 3/4 = 0.75. another ex, r=6, r/s=6/4=1.5, r=9, r/s=9/4=2.25. Thus, any integer divided by 4 always gives a terminating decimal. Hence (II) is SUFF and answer is (b).

What if r = 8 or any multiple of 4, then it r/s can't be a terminating decimal?

Comment please....

codesnooker,
Please, look at the examples given in the stem. "for example, 24, 0.82, 5.096 are three terminating decimals" 24 is in the list, so multiples of 4 are terminating decimals according to the given definition.

My bad! I missed that part. Thanks for the notification. [:)]

Whether a fraction will be terminating or not is solely dependent on the value (prime factors) of denominator in when the fraction is expressed in simplest form. If in the simplest form (no common factor between numerator and denominator) the denominator only consists of 2 & 5 as prime factors, the fraction will always terminate.

So to find out whether the fraction will be terminating or non-terminating
1. Remove common factors from numerator & denominator
2. Express denominator in terms of prime factors.
3. If you find any prime factor other than 2 or 5, then the fraction is non-terminating....