Yes, we can calculate the smallest
supernet that contains 2 given networks (or any number of networks).
The easiest way to do this is in binary (yes really):
First convert your two addresses in binary:
10.1.192.0 = 00001010.00000001.11000000.00000000
10.1.240.0 = 00001010.00000001.11110000.00000000
As you can see, the part that is common to this two addresses is 00001010.00000001.11 which is 18 bits long.
So we now we are looking for a /18 network.
To get the network address we take those 18 bits and complete to 32 bits with trailing zeros and we get:
00001010.00000001.11000000.00000000 /18 = 10.1.192.0 /18
Now to calculate your Y value, assuming subnet of equal size, i.e. /20, this boil down to "how many /20 subnet do we have in a /18 network".
To calculate this we the take the size difference and square it.
20 - 18 = 2
2^2 = 4 = Y
=> we have 4 /20 subnets.