The cost of children’s ticket is $ 5
Solution:
Let "c" be the cost of one children ticket
Let "a" be the cost of one adult ticket
Given that adult ticket to a museum costs 3$ more than a children’s ticket
Cost of one adult ticket = 3 + cost of one children ticket
a = 3 + c ------ eqn 1
Given that 200 adult tickets and 100 children tickets are sold, the total revenue is $2100
200 adult tickets x cost of one adult ticket + 100 children tickets x cost of one children ticket = 2100

200a + 100c = 2100 ------ eqn 2
Let us solve eqn 1 and eqn 2 to find values of "a" and "c"
Substitute eqn 1 in eqn 2
200(3 + c) + 100c = 2100
600 + 200c + 100c = 2100
600 + 300c = 2100
300c = 1500
c = 5
Thus the cost of children’s ticket is $ 5