What XML tag is in correct and makes this document not well$-$formed?

$12$

$28897$

$10/01/2020$

$10/05/2020$

$124789$

$1$

$12$

$845-449-5847$

$1$

$20\mathrm{.}50$

$369729$

$2$

$12$

$429-331-1516$

$1$

$15\mathrm{.}99$