7.3 ------ 8.1 8.5 8.9
let us try to find the common difference here using last three terms
8.5-8.1=0.4
8.9-8.5=0.4
so it is clear that common difference is 0.4
so adding 0.4 to 7.3 will give the next term
7.3+0.4=7.7
Answer : missing number is 7.7
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