987 plus 100 equals what?
This question is asking us to find 100 more than 987, which is a sort of calculation that we can work out mentally. 987 is the same as 900, 80, and seven. These are the values of the hundreds, tens, and ones digits. When we add 100 to a number, the tens and the ones stay the same. The hundreds increase by one. 900 plus 100 equals 1000. And so 987 becomes 1087.
We found the answer mentally. First, we partitioned the number 987 into its hundreds, tens, and units: 900, 80, and seven. A quicker way of partitioning the number might have been just to separate off the hundreds: 900 and 87. We know that adding 100 means that we have to increase the number of hundreds by one. And so we knew that 900 becomes 10 lots of 100, or 1000. We then combined 1000 and 87 to give us our answer. 987 plus 100 equals 1087.