Let $|G|=80$ be an abelian group. Find the number of elements of order $20$ in $G$ given there’s more than $33$ of those, and no element of order $40$

abelian-groupsabstract-algebrafinite-groupsgroup-theorysylow-theory

Let $G$ is an abelian group of order $80$, given that there is more than $33$ elements of order $20$, and given $G$ doesn't have an element of order $40$, find the number of elements of order $20$.

My Attempt:

As $G$ is abelian every subgroup of $G$ is normal. Also, $G$ has a $2$-Sylow subgroup and a $5$-sylow-subgroup. As every subgroup of $G$ is normal, this $p$-sylow subgroups are normal and therefore ${\rm Syl}_2=1$ and ${\rm Syl}_5=1$.

Because $16= 2^4$ is a divisor of $80$ and $2^5=32$ is not, we know there exists a subgroup of order $4,8,16$ in $G$.

Also, for every $m>2$ the product of every $m$ subgroups of $G$ is also a subgroup (again, a normal subgroup as well) of $G$.

All this haven't gotten me closer to understand and find the number of elements of order $20$ in $G$, and also I can't understand how to use the argument that there's no element of order $40$ in $G$.

Best Answer

We must have $C_4×C_4×C_5$, because otherwise we have fewer than $33$ elements of order $20$.

Thus we count $48$, since there are $12$ elements of order $4$ in $C_4×C_4$..