Oscar A is estimating the answer for you, since he happens to know that 552 = 3025. However, he is not bounding the answer (i.e., determining lower and upper limits). To do so, round the original factors down and up. To round them down, try 5*300 = 1500, and to round them up, try 10*400 = 4000. Therefore, we know that the answer lies between 1500 and 4000. Unfortunately, this is a very big range. To be more precise, you can try 8*300 = 2400 and 8*400 = 3200. To be even more precise, you can try 8*370 and 8*380.