Proabably. 5 because if there is 2 sides on a coin then you should divide 10÷2 which is 5.
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Pineapple juice : orange juice : lime juice = 2:7:3
So there is 2x of pineapple juice, 7x of orange juice and 3x of lime juice.

She used 100 ml of pineapple juice.
Common difference r will be the difference between next two values of number row, so we take e.g. second minus first: r=9.4-6=3.4. You can see, that this value is constant as we go through next pairs of values.