An alkene $'A'$ on ozonolysis gives a mixture of ethanal and pentan$-3-$one. Write the structure and $IUPAC$ name of $'A'$.

(N/A) The ozonolysis of an alkene involves the addition of ozone to the double bond to form an ozonide,followed by reductive cleavage using $Zn/H_2O$ to yield carbonyl compounds.
Given products are:
$1$. Ethanal: $CH_3-CHO$
$2$. Pentan$-3-$one: $CH_3-CH_2-C(=O)-CH_2-CH_3$
To find the structure of the alkene $'A'$,we remove the oxygen atoms from the carbonyl groups and join the carbon atoms with a double bond:
$CH_3-CH=O + O=C(CH_2-CH_3)_2 \rightarrow CH_3-CH=C(CH_2-CH_3)_2$
The structure of $'A'$ is $CH_3-CH=C(CH_2-CH_3)_2$.
The longest carbon chain containing the double bond has $6$ carbons. Numbering from the right to give the double bond the lowest position:
$CH_3(1)-CH_2(2)-C(3)(CH_2-CH_3)=CH(4)-CH_3(5)$ is incorrect; the longest chain is $6$ carbons long: $CH_3-CH_2-C(=CH-CH_3)-CH_2-CH_3$.
The $IUPAC$ name is $3-$ethylhex$-2-$ene.