Arithmetic selectivity is one of the advantages of a superheterodyne receiver. What is means is that a superheterodyne receiver is more selective (has a smaller band of frequencies that it will accept, process and amplify) for the same bandwidth percentage (bw %) than a conventional receiver. The secret to this argument is that the superhet always operates at a lower frequency (IF) than the conventional receiver. If the receiver bw % remains the same, the range of frequencies is always less for the IF because you are multiplying bw % times a smaller number (the IF). Here's an example that should clear it up:
Suppose you are tuned to an AM radio station at 1100 kHz. Your receiver has a bw % = 3%. That means it will accept all frequencies from 1067 - 1133 kHz ( 1100 +/- 3%). If you have an interfering signal at 1125 kHz, then your receiver will accept it, too.
Here's where a superhet receiver can be useful. Without changing the bw %, the superhet receiver can reject the interference. Assume that the superhet uses a fixed IF of 455 kHz (this is true for all AM superhet receivers). It will pass and amplify all signals in the band 455 +/- 3% = 433 - 477 kHz (see figure). When you tune the superhet receiver, you mix the incoming signal with 655 kHz (or 1555 kHz) to create the 455 kHz (center) signal. The interfering signal will also mix with the 655 kHz and be reduced to 480 kHz (still 25 kHz above the desired signal). That is now outside of the operating band of the IF receiver and will be rejected.
The fact that a superheterodyne receiver passes a smaller range of frequencies for a fixed bandwidth percentage is called arithmetic selectivity and is one of its major advantages over conventional receivers.