Greek words related to number "fifteen":
Greek word DEKAPENTE means "fifteen", and DEKATOS PEMPTOS means "fifteenth", [Divry's Modern English-Greek, Greek-English Dictionary, 1988, p. 762].
Turkish cardinal numeral for "fifteen" is "ONBEŞ" and for ordinal numeral "fifteenth" is "ONBEŞINCI".
a) The Greek word DEKAPENTE, meaning "fifteen", when rearranged in two parts as "DEKA-PENTE", we have the two terms DEKA and PENTE which were explained under Item 12 -Greek words related to number "ten" and under Item 7 - Greek words related to number "five" respectively. There we found that DEKA was part of Greek words which were made up from Turkish expressions that used Turkish numeral ON, and similarly, PENTE was part of Greek words which were made up from Turkish expressions that used Turkish numeral name BEŞ. Thus, the term DEKA gets its meaning from Turkish ON, while PENTE gets its meaning from Turkish BEŞ. With these findings, the Greek cardinal numeral DEKAPENTE gets its meaning from Turkish cardinal numeral name "ONBEŞ" meaning "fifteen".
b) The Greek ordinal numeral composite name DEKATOS PEMPTOS, meaning "fifteenth", when rearranged as "OS-KATE-PESMTDO-P", is the altered, restructured, disguised and Hellenized form of the Turkish mathematical expression "ÜŞ KATI BEŞiMCiDU" (ÜÇ KATI BEŞiNCiDU)", i.e., (3x5) INCIDI = 15 INCIDI" meaning "it is (three times five)th", i.e, (3x5)th = 15th. Thus, in this case, a mathematical expression in Turkish plus the suffix INCIDI, rather then the Turkish ordinal numeral name ONBEŞiNCi, have been used in the makeup of the Greek ordinal numeral DEKATOS PEMPTOS. Hence, its source is from Turkish.