For domestic use, powered by 13,500 lb (60.5 kN) Pratt & Whitney JT3C-6 turbojets with water injection. The initial DC-8-11 model had the original, high-drag wingtips and all were converted to DC-8-12 standard. The DC-8-12 had the new wingtips and leading edge slot, 80 inches long between the engines on each wing and 34 inches long inboard of the inner engines. These unique devices were covered by doors on the upper and lower wing surfaces that opened for low speed flight and closed for cruise. The maximum weight increased from . 28 DC-8-10s were built. This model was originally named "DC-8A" until the series 30 was introduced. 29 built, 22 for United and 6 for Delta, plus the prototype. By the mid sixties United had converted 15 of its 20 surviving aircraft to DC-8-20 standard and the other 5 to -50s. Delta converted its 6 to DC-8-50s.
Question: How many more DC-8-10 were built for United than Delta?
Answer: 23