To estimate each number by rounding it to the nearest ten, we will look at the unit digit,
If it is less than 5, then we replace it by 0 and keep the ten-digit as it
If it is 5 or more, then we will replace it by 0 and add the ten-digit by 1
Let us do that with every number
28, the unit digit is 8 which is greater than 5, then replace it by 0 and add 2 by 1
28 rounded to 30
51, the unit digit is 1 which is less than 5, then replace it by 0
51 rounded to 50
76, the unit digit is 6 which is greater than 5, then replace it by 0 and add 7 by 1
76 rounded to 80
Now let us multiply them
The product of the given numbers is 120,000