Answer:
40% increase
Explanation:
from 15 to 21 is an increase
old value = 15
new value = 21
formula is: (new value - old value) / old value x 100
[(21 - 15) / 15] x 100 = 6/15 x 100
= 2/5 x 100
= .4 x 100
= 40%
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