To provide an estimate we can see that if the question had been 48000-28000, the answer would have been 20,000. 47,302 is close to 48,000 and 28,113 is close to 28,000. Or we could use the fact that 47,302 is close to 50,000 and 28,113 is close to 30,000. 50,000-30,000=20,000. So our estimate is still 20,000.
That means, when we do the accurate arithmetic we expect the answer to be around 20,000.
In fact, the accurate answer is 19,189, so the estimate was quite good!