50/50 ratio by volume

Pure Pb bullet is 1.59cm3, unknown bullet is same volume

50% Pb by volume in unknown mix is .795cm3 x 175.03gr/cm3 = 139.14gr
50% Sn by volume in unknown mix is .795cm3 x 112.12gr/cm3 = 89.13gr

Pb + Sn = 228.27gr for approx same volume bullet