IMSO 2014 Science International — Theory I Question 10

Question 10

A force $F$ is acting on a mass of $m_1$ resulting in an acceleration of $a_1$. The same force is acting on a different mass of $m_2$ resulting in an acceleration of $a_2=2a_1$.

Question illustration

If $m_1$ and $m_2$ are glued together and the same force $F$ acts on this combination, what is the resulting acceleration?

$\frac13a_1$
$\frac23a_1$
$\frac43a_1$
$\frac32a_1$