From Rav Moshe zt”l’s first teshuvah listed here [Fig. a] we see he maintained that if the tzuras hapesach encircled fewer than shishim ribo it is considered as if the tzuras hapesach separates the neighborhood from the whole city and an eruv is permissible. Additionally, both these teshuvos [Figs. a and b] demonstrate that Rav Moshe was led to believe that independently Boro Park and Flatbush have populations of shishim ribo and therefore an eruv of tzuras hapesachim was not permissible. Rav Moshe states that each one of these neighborhoods encompasses less than twelve mil by twelve mil and nevertheless they have more than shishim ribo. On the other hand Rav Moshe states that Brooklyn includes an area of more than twelve mil by twelve mil (Igros Moshe, O.C. 4:87-88). Consequently, these teshuvos must indicate that each neighborhood independently has shishim ribo and not just the whole of Brooklyn. Therefore, since the Boro Park and Flatbush eruv contain considerably less then shishim ribo inhabitants (approximately 100,000 and 200,000 respectively − NYC Department of City Planning, Community District Profiles, 2002) there is no reason according to Rav Moshe not to allow an eruv to be established (see also Brooklyn and Queens: Same or Different?).