$1$

$2.$ Diffraction measurements on the CaO$-$SiO$2$ glass represented by Figure $4\mathrm{.}22$ show that the nearest neighbor $Ca2+-Ca2+$ distance, $R1,$ is $0\mathrm{.}375nm.$ What would be the next$-$ nearest neighbor $Ca2+-Ca2+$ distance, $R2?$