Manila makes a drum in 4 weeks = 1/4 drum a week.
Zane makes a drum in 6 weeks = 1/6 drum a week.
X - number of weeks to make 15 drums.
15 = X(1/4 + 1/6)
X = 15/(1/4 + 1/6) = 15/(3/12 + 2/12) = 15/(5/12) = (15/5) x 12 = 3 x 12 = 36 weeks
check: 15 = X(1/4 + 1/6) = 15 = 36(1/4 + 1/6) = 15 = 36/4 + 36/6 = 9 + 6 = 15