Harper inherited a debt of about $500 billion

You have to pay bank interest on that debt

Let's say  at  5% interest per year that would be  $25 billion per year

In 9 years of Harper administration that would be $225 billion of interest that Harper paid

It is said Harper added only $150 billion or so to the debt,
 which means Harper added NOTHING  to the national debt 
because the addition to the debt was basically the interest growing