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65536 mold spores

Step-by-step 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