The "integer core" of a number is found by adding the digits

The "integer core" of a number is found by adding the digits of a given number continuously until there is a
the "integer core" of a number is found by adding the digits of a given number continuously until there is a single digit remaining.for example,the integer core of 3577 is 4 because 3+5+7+7=22 and 2+2=4.what is the largest number less than 1,000 with an integer core of