Answer
$y^{20}$
Work Step by Step
Using $x^a \cdot x^b=x^{a+b}$ or the product rule, the given expression, $
y\cdot y^{19}
,$ simplifies to
\begin{array}{l}\require{cancel}
y^{1+19}
\\\\=
y^{20}
.\end{array}
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