To Estimate the sum of 405 and 385, we will add 405 together with 385 I.e 405+385 to get 790.
Rounding 790 to a hundred is 800 since the second place value in 790 which is '9' is greater than 5, we will add 1 to the first place which is 7 to give us 8. This will change the second place value to '0' which make the value of 790 to be 800 to a hundred.
Note that 1 can only be added to a preceding value of a number if the succeeding value is greater than 5.
