A minute to spare

The test had twenty questions and forty minutes, so every question could have two minutes. I worked steadily and wrote my last answer after thirty-eight minutes.

$20$ questions, $40$ min, so $40÷20=2$ min each. I wrote my last answer at minute $38$.

Questions $Q$, time $T$ min: $Q=20,T=40$, and $T÷Q=2$. Last answer at $38$.

I had two minutes to spare. Most of the class had already handed in their papers, and I wanted to do the same. Then I remembered what my teacher always says: a test is not finished until you have checked it.

$40-38=2$ min to spare. Others had handed in; I wanted to as well. But a test is not finished until it is checked.

Spare: $40-38=2$. Check first, then hand in.

I read my answers again and found one that was wrong. I had added instead of multiplying: six plus seven instead of six times seven. Putting it right took one minute.

Checking, I found one wrong answer: I had written $6+7=13$ instead of $6\times 7=42$. Putting it right took $1$ min.

Check: $6+7=13$ wrong, $6\times 7=42$ right.

Each question was worth five marks. Before checking I had fifteen right, which is seventy-five marks out of a hundred. After checking I had sixteen right, which is eighty. One minute of checking was worth five marks.

Each question $5$ marks. Before checking: $15\times 5=75$ of $100$, that is $75\%$. After: $16\times 5=80$, that is $80\%$. One minute of checking, $5$ marks.

Marks per question $5$. Before $M_0=15\times 5=75$, after $M_1=16\times 5=80$, so $M_1-M_0=5$.