Chem Help!

The presence of 11-cis retinal, C20H28O, in cells in the eye is important for vision.
The structure of retinal includes an aldehyde group, a cyclohexene ring and a long aliphatic side
chain, in which a carbon-carbon double bond exists between carbons numbered 11 and 12.
Which pair of statements about 11-cis retinal could be correct?

5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite sides side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite side

I automatically knew that the R1/R2 hydrocarbon groups had to be on the same side because it's a cis compound, which eliminates two of the answers, but then how do you figure out the number of C=C double bonds with only that information and the molecular formula?

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  1. For 20carbon means that there are 40Hyrdrogen.
    Number of hydrogen left =40-28=12
    To make an aldehyde, 2H is used
    Number of hydrogen left after aldehye is made = 12-2 = 10
    To make alkene (double bond), 2H is used for one double bond
    Since there are 10hydrogen left,
    10/2=5
    There are 5 C=C and C=O

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  2. Why divide by 2. And why are 40 carbons there r 28 of them and if there are 40 why subtracted 28!!!!!!!

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  3. (1)For an alkane which only has a straight main chain with 20 carbon atoms, there will be 20*2 +2 = 42 hydrogen atoms. (2) When adding a ring into the main chain, subtract 2 hydrogen atoms. This is because when adding the ring, you are actually adding two alkyl groups; consequently, the two previous hydrogen atoms boning to the carbon on which you add a ring are actually replaced by the two 'alkyl groups'. (3) When having an aldehyde, two hydrogen atoms are removed, with one lost due to the forming of a double bond between a carbon atom and an oxygen atom, and with one lost owing to the oxygen atom. (4) When adding an alkene functional group, two hydrogen atoms are lost, as can be demonstrated by the fact that ethane has six hydrogen atoms in total but ethene only has four hydrogen atoms. (5) Hence, according to the aforementioned four reasons, we can firstly subtract 2 H from the 42 hydrogen atoms due to the presence of an aldehyde group; secondly, we subtract 2 H form the remaining 40 hydrogen atoms, as a cyclohexene ring presents; and then we know that there are 38-28=10 hydrogen atoms lost because of the presence of alkene groups. Therefore, divide 10 by 2; the result is 5, meaning that there are five double bonds (c=c).

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