user: Question: What is the highest Quantity for PTRD Nos. 3, 15?
Columns: ['Manufacturer', 'Type', 'Quantity', 'PTRD Nos.', 'GWR Nos.']
Types: ['text', 'text', 'real', 'text', 'text']
Sample row: ['Robert Stephenson & Co.', '0-6-2T', 7.0, '8–14', '183–187']
SQL:

assistant: SELECT MAX("Quantity") FROM "Table" WHERE "PTRD Nos." = '3, 15';