N. Siregar and H. Haniza, “Economic Age of Boilers of Oil Palms (Elais) Factory of North Sumatra, Indonesia”, IJSBAR, vol. 36, no. 8, pp. 138–156, Dec. 2017, Accessed: Feb. 09, 2026. [Online]. Available: https://www.gssrr.org/JournalOfBasicAndApplied/article/view/8469