The sum of sigma $(\sigma)$ and pi $(\pi)$ bonds in $Hexa-1,3-dien-5-yne$ is $...........$.

The order of priority of the following functional groups in the $IUPAC$ method of naming organic compounds is:
$(i) >C=O$
$(ii) -NH_2$
$(iii) -CN$
$(iv) -COOR$

The $IUPAC$ name of the compound $CH_2 = CH - CH(CH_3) - CH_3$ is

What is the $IUPAC$ name of the given compound?

Fill in the blanks:
$(1)$ The $IUPAC$ name of $CH_3COCH_2CH_2CH_2COOH$ is .........
$(2)$ The formula of pent$-4-$en$-2-$ol is .........
$(3)$ The $IUPAC$ name of the following structure is ......... and ......... is the main functional group in it.
(Structure: $A$ cyclohexene ring with an $-OH$ group attached to the carbon adjacent to the double bond,such that the double bond is between carbons $2$ and $3$ relative to the $-OH$ at position $1$.)
$(4)$ For $CH_3CH(OH)CH_2CH_2CH_2CHO$,the main functional group and the second priority group are ......... and ......... respectively.

What is the correct $IUPAC$ name of the following structure? Explain.
$CH_3-CH_2-CH(CH_2-CH_3)-CH_2-CH_2-CH(CH_3)-CH_2-CH_3$
$(i)$ $3-$ethyl$-6-$methyloctane
$(ii)$ $3-$methyl$-6-$ethyloctane
$(iii)$ $6-$methyl$-3-$ethyloctane