An old man was jogging around a 400 $$m$$ running track.<br>During the firstvtwo rounds, he jogged slowly and completed it in 13$$\frac{1}{2}$$ minutes.<br>For the next two rounds, he jogged faster and completed them in 6$$\frac{1}{2}$$ minutes. <hr> What was his average speed forth four rounds that he ran in $$m/min$$? <br> <input min="0" type="number" id ="imp6pg158q4_1" class="lineinput" placeholder="?" /> $$m/min$$