Answer:
900
Explanation:
I'm gonna define the "missing number" as X, so it's easier.
3947 = 3000 + 40 + 7 + X
First, add all the numbers
3947 = 3047 + X
Now you've got that, to get the X value you've to leave X alone in a side of the equatioin, in our case we can subtract 3047 from each side. What ever you do in a side you've to do it the same at the other one.
3947 - 3047 = 3047 - 3047 + X
900 = X
Now we know the missing number is 900. Not sure about it? replace every X in the first equation with 900.
3000 + 40 + 7 + 900 = 3947, and that's correct.