Answer
The internal energy of the hands has increased by $2.9~J$
Work Step by Step
We can find the magnitude of the work done by friction:
$Work = 8\times F~\mu_k~d$
$Work = 8\times (5.0~N)(0.45)(0.16~m)$
$Work = 2.9~J$
We can assume that the magnitude of the work done by friction is converted into internal energy in the hands. The internal energy of the hands has increased by $2.9~J$
