Estimated product is found by using numbers that are close to the ones you are multiplying together. The idea is to make the multiplication easier. In this case 19 is close to 20, so we use 20 instead of 19. 78 is close to 80, so we estimate the product by multiplying 20 and 80. The answer is 1600, so we expect the real product of 19 and 78 to be round about 1600. A bit less really, because 20 and 80 are both less than 19 and 78.
Let’s see what the answer is and compare with our estimate: 1482 compared with an estimate of 1600.
Another estimate is to choose a convenient number more than 19 and a number less than 78, because the product should be closer, the excess (bigger than 19) balanced by the deficiency (smaller than 78). This time we could pick 20 and 75 and the product estimate is then 1500, closer to the right answer.
Yet another estimate would be to pick 15 and 80, because 15 is a little less than 19 and 80 is a little more than 78. We get 1200.
The thing about estimates is that there is no right or wrong answer as long as you make a sensible guess at the substitute numbers. The best estimate we found was 20 and 75, giving 1500 as the rough product. Always look for numbers you can multiply in your head.