If I were to answer this, I will use 2 equation. First, let x be the number of adults and y be the number of children. Where : x+y=40 and 22x+18y=820 . Let us use the substitution method. We'll use the first equation because it's more simple. If x+y=40, then x=40-y . Then, let's substitute the value of x in the equation number 2. If 22x+18y=820, then, 22(40-y)+18y=820. Simplifying it, we'll get the value of y which is 15. Then, let's substitute the value of y to the first equation to get the value of x which is 25. Therefore, if x = 25 and y = 15, there should be 25 adults and 15 children in the group.