There are 10-pound weights and 40-pound weights and the total weight of the set is 850 pounds. 
The given equation is:

We know that 850 represents the total weight of the set. Thus:
10n represents a weight, and 40(12) also represents a weight. 
The term 40(12) in the equation, tells us that there are twelve 40-pound weights and 40(12) is the weight of those 12 weights. 
The term 10n in the equation, tells us that there are "n" 10-pound weights, and 10n is the total weight of those n 10-pound weights. 
When we find the solution for "n" we will have found the number to 10-pound weights in the set. 
Answer: 
The number of 10 lb weights in the set