a = 2 min
b = 8 min

a) $\frac{2}{8}\ \frac{0.25 min}{15 sec}$
= 8 min 15 sek

b) 8 min  * 0.8 + 2 min = 8,4 minuten

(a+b)/ 8,25 = 1,21111
(a+b)/ 8,4 = 1,19