We are estimating, so we round the numbers to something that would be beneficial to us to more easily solve the problem (using mental math). We want to round to compatible numbers to make the math more simple. If we round to numbers that are divisible by 10 it makes the association easy to see... If we round 276 to 280 and 42 to 40, we see that we have complimentary numbers because 28 divides evenly by 4 to give 7. So
280 / 40 = 7
