Answer:
C = 580 children. 
A = 420 adult. 
Explanation:
Let the number of children be C. 
Let the number of adult be A. 
Translating the word problem into an algebraic equation, we have;
A + C = 1000 .......equation 1
10A + 5C = 7100 .......equation 2
A = 1000 - C .......equation 3
Substituting eqn 3 into eqn 2, we have;
10(1000 - C) + 5C = 7100
10000 - 10C + 5C = 7100
10000 - 7100 = 10C - 5C
2900 = 5C
C = 2900/5
C = 580 children. 
Now, to find the value of A. 
From equation 3;
A = 1000 - C
A = 1000 - 580
A = 420 adult.