The first thing we must do for this case is find the factors of each of the numbers.
 We have then:
 60: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60
 90: 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
 Therefore the GCF is given by:
  
 So, by rewriting the sum we have:
  
 The error is not having written the GCF multiplying.
 Answer:
 Xiao did not write the sum as the product of GCF and the sum of two numbers.
 The correct expression is:
 