Question 572741
This athlete trains for 60 minutes.  Some of those minutes are spent walking and some are spent running.


X minutes he spends running.  The rest of the hour (60-X) minutes he spends walking.


If he burns 10 cals per minute running then his total calories running are 
10 cals/min times X minutes.  
 
If he burns 4 cals per minute walking then his total calories walking are 4 cals/min times (60-X) minutes.

All together he burns 450 calories.


Put these 3 statements together and we have 
10X + 4(60-X) = 450
10X + 240 - 4X = 450.  Subtract 240 from each side of the equation ... 
10X - 4X = 210.  Combining like X terms ... 
6X = 210.  Dividing both sides by 6 ... 
X = 35 minutes


He spends 35 minutes running and 60-35 = 25 minutes walking.


Let's check our answer:
10 cals/min times 35 minutes = 350 calories.
4 cals/min times 25 minutes = 100 calories.
Adding them gives us 450 calories.  Check!