ISRO-previous-paper-2017-Dec Computer Science question 3,Set A

❔ A bag contains 19 red balls and 19 black balls. Two balls are removed at a time repeatedly and discarded if they are of the same colour, but if they are different, black ball is discarded and red ball is returned to the bag. The probability that this process will terminate with one red ball is

a) 1
b) 1/21
c) 0
d) 0.5

Answer is 1:

there are 4 possible combinations of balls:

RR -- discard
RB -- keep red discard black
BR -- keep red discard black
BB -- discard

repeated combination comes for each above (RR,RB,BR,BB) then,
due odd number of balls:
the last combination in the bag is RB.

ISRO previous paper 2017 December, Computer Science question 13, set A

❔ Consider the following schema :

Sailors(sid,sname,rating,age)
Boats(bid,bname, colour)
Reserves(sid,bid,day)

Two boats can have the same name but the colour differentiates them.
The two relations

 ρ(Tempsids, (π_{sid, bid} Reserves) / (π_bid ( σ _{bname='Ganga'} Boats))),
π_sname(Tempsids ⋈ Sailors)

If / is division operation, the above set of relations represents the query
(a) Names of sailors who have reserved all boats called Ganga
(b) Names of sailors who have not reserved any Ganga boat
(c) Names of sailors who have reserved at least one Ganga boat
(d) Names of sailors who have reserved at most one Ganga boat

Answer is (a),

best refer to you tube video to understand the concept of division operation in DBMS.

Division Operator in DBMS Relational Algebra with Example in English, Hindi