Write a question about the following article: Sydney has been a manufacturing city since the protectionist policies of the 1920s. By 1961 the industry accounted for 39% of all employment and by 1970 over 30% of all Australian manufacturing jobs were in Sydney. Its status has declined in more recent decades, making up 12.6% of employment in 2001 and 8.5% in 2011. Between 1970 and 1985 there was a loss of 180,000 manufacturing jobs. The city is still the largest manufacturing centre in Australia. Its manufacturing output of $21.7 billion in 2013 was greater than that of Melbourne with $18.9 billion. Observers have noted Sydneys focus on the domestic market and high-tech manufacturing as reasons for its resilience against the high Australian dollar of the early 2010s.

Question about the article:
How many billions of dollars did Sydney and Melbourne combine output in manufacturing in 2013?