The value of an underlined digit is calculated by multiplying its place value by the value of the single digit. Place value is defined as the value of the place of a digit in a number.
In a number, place values from right to left are ones, tens, hundreds, thousands, ten-thousands, hundred-thousands, millions and so on. By definition, a place value number is a 1 followed by a number of zeros. An example calculation of the value of an underlined digit would be if the digit 4 were underlined in 1040, the underlined value of that digit would be 40.