Variables in t() causes repeated translation strings

The following line:

'#title' => t('Math Question: What is '. $x .' + '. $y .'?'),

should be replaced by something like:

'#title' => t('Math Question: What is %problem?', array('%problem' => $x .' + '. $y)),

to avoid multiple copies of the same translation string appearing with varying values of $x and $y.