Answer:
10×6 with 20% added = 60+12 = 72
Explanation:
There are several ways to estimate the product of these numbers. One way is to round 12 to 10, then multiply 10×6 to get 60 as an estimate. You know this estimate is low, because you rounded down from 12 to 10. You know that a better estimate will be about 20% higher, because 12 is 20% higher than 10.
The product can be computed mentally by recognizing that 12 is 6×2, so the product will be 6²×2. Most schools have you memorize your times tables at least through 10×10, so you already know that 6² is 36. Doubling that value (to 72) gives you the value of 6×12.
Another way to estimate the product is to round 12 to 10 and 6 to 5. Again, you know each of these "stand-in" values needs to be increased by 20% to match the real values. Then the product estimate is 10×5 = 50, and you judge that to be low by 20% +20% = 40%. 40% more than 50 gives you a refined estimate of 70, but you know that, too, is low by a small amount. (Percentage changes can only be added accurately when they are very small. The true error is 44%, which is 20%+20%+(20%)(20%).)