Answer:
12 hours
Step-by-step explanation:
Create a proportion where x is how long the 21 cm candle can burn for:
[tex]\frac{14}{8}[/tex] = [tex]\frac{21}{x}[/tex]
Cross multiply and solve for x:
14x = 168
x = 12
So, the 21 cm tall candle can burn for 12 hours