Question 33715
1.) first, define the variables:
s=number of orders under 10 pounds
m=number of orders between 10 and 15 pounds
L=number of orders more than 15 pounds
2.) now, let us produce the three essential equations:
3s + 5m + 7.5L = 680 
the cost times the amount of orders will equal the total cost
s - 3L = 0
there were three times as much orders under 10lb than there were above 15lb
s + m + L = 150
there were 150 orders in all
3.) next, you combine equations to get:
5m + 16.5L = 680
and
m + 4L = 150
4.) then, combine these to get:
L = 20
5.) now, plug L into one of the equations mentioned in step (3) and you get:
m = 70
6.) last, plug in m and L in one of the equations mentioned in step (2) and you get: s = 60
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the answer is: s=60 while m=70 while L=20