Answer:
65536 mold spores
Explanation:
After 3 hours , 4² mold spores are growing.
After 6 hours , 4³ mold spores are growing.
After 9 hours , 4⁴ mold spores are growing.
After 12 hours , 4⁵ mold spores are growing.
It means in every 3 hours , 4 new spores form and grow from each pre-existing spore.
⇒ After 15 hours , 4⁶ mold spores are growing.
⇒ After 18 hours , 4⁷ mold spores are growing.
⇒ After 21 hours , 4⁸ = 65536 mold spores are growing