Simplify the following text for a Grade 10 native English-speaking person.
Original: Although total worldwide album sales are not calculated by any single entity , as of 2004 several industry sources estimated Rush 's total worldwide album sales at over 40 million units .
Simplified: Although total worldwide album sales are not calculated by any single person or group , as of 2004 some people guessed that Rush 's total worldwide album sales is over 40 million items sold .