The answer is 1/30
The slips contain the first 11 prime numbers. Thus the numbers on the slips are as below:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
We draw out 4 slips from the given 11.
Thus the total number of ways 4 numbers can be drawn is 11C4 = (11*10*9*8)/(4*3*2*1) = 330
Now we need to find the total favourable number of draws.
For range to be 12, the biggest of the 4 numbers and the smallest of the 4 numbers should have a difference of 12
If we look at the above list of numbers, the below combinations yield that result
(5,17), (7,19), (11, 23), (17,29) and (19,31)
Now lets try to figure out the number of possible draws for each pair
For (5,17):
Now we know that out of the 4 slips drawn, two will be 5 and 17. For the other two, the numbers cannot be greater than 17 and less than 5 (as the moment that happens, the range will change). Thus, the two remaining numbers will be between 5 and 17. The same will apply for all pairs.
Between 5 and 17 there are 3 possible numbers and we have to pick 2 i.e. 3C2 = 3 draws
For (7,19):
Again we have 3 possible numbers out of which we have to pick 2 i.e 3C2 = 3 draws
For (11,23):
Same as the above 2 = 3 draws
For (17,29):
Here there are 2 numbers only between 17 and 29. Hence, we have to pick 2 numbers from 2 possible options i.e. 2C2 = 1 draw
For (19,31):
Same as above = 1 draw
Thus, total possible favourable draws = 3+3+3+1+1 = 11
Hence the required probability = 11/330 = 1/30