Answer:
0.06
Explanation:
let 0.09 be a1
and 0.15 be a2
and 0.21 be a3
common difference = a2-a1 or a3-a2
a2-a1= 0.15-0.09 = 0.06
a3-a2= 0.06
hence the common difference in the ap in 0.06
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