If the ratio of 13C to 12C is 1.08% (1.07/98.93), then a compound with ten carbons would have a ratio of 10.8%. So if the ratio is 21.15/85.03, then how many carbons are in the compound? (21.15/85.03)/1.08 = 0.230310871 or 23 carbons. 23*1.08 = (21.15/85.03) = 24.84