Question 1129628
Your reasoning is correct, D has a lower daily training time because including D brings the average time down. <br>

We can find D's time as follows:

The average of A,B,C is found by adding their times and dividing by 3:
(A+B+C)/3 = 12.2    (eq 1)<br>

The average of A,B,C,D is found by adding their times and dividing by 4:
(A+B+C+D)/4 = 11.5   (eq 2) <br>


In (eq 1), multiply both sides by 3:    
3*(A+B+C)/3 = 3*12.2     <—— the left hand side will become 1*(A+B+C) or just A+B+C
A+B+C = 36.6<br>

In (eq 2), multiply both sides by 4:    
4*(A+B+C+D)/4 = 4*11.5
A+B+C+D = 46 <br>

Now notice we can substitute 36.6 for "A+B+C" in this last equation:  
36.6 + D = 46<br>

Finally, subtracting 36.6 from both sides:
36.6 + D - 36.6 = 46 - 36.6
   D = 46-36.6 
{{{ highlight(   D = 9.4 ) }}}