<p>Since we do have a couple of suggestions, I'll add mine that is a mere variance. However, if this were on the SAT, this how I would do since I WANT to remember that lengthy problem most often have the simplest of solutions! </p>
<ol>
<li>Difference of rates between clocks is 2 - (-4) = 6 minutes</li>
<li>Total Difference at the end is 8 - 7 = 1 hour or 60 minutes</li>
<li>Numbers of hours needed to account for 60 minutes difference is 60/6 = 10.</li>
</ol>
<p>Now, we know that ten REAL hours are needed, so let's take a look at both clocks; </p>
<p>In ten real hours, the fast clock would have marked 10 x 62 or 620 minutes. So, the initial time was 8 - 10h20 or 9h40. </p>
<p>Time to solve this? about 30 seconds and 15 if you used military time! :)</p>
<p>PS The only numbers on my paper were 6 ... 60 ...10... 620...2000 ...1020 ... 940 </p>
<p>PPS You could have taken the slow clock and done 10 x 56 or 560 minutes. The initial time would be 7 - 9h20 or 9h40.</p>