Simply it as follows:
1 + 39 = 40
2 + 38 = 40 etc
18 + 22 = 40
19 + 21 = 40
There are 19 of these plus 40 itself . . .therefore 20 of them
20 x 40 = 800
And finally the last number not used, 20
Therefore 800 + 20 = 820
Dang I was just about to say 22
3x + 4y = 31
2x - 4y = -6
*The +4 and -4 in the center of the equation cancel out because 4-4 = 0*
3x = 31
2x = -6
------------
*add like terms*
3x + 2x = 5x
31 - 6 = 25
So now you should have this written on your paper ... > 5x = 25
*Divide by 5 on each side*
5x = 25
_ _
5 5
x = 5
Are you sure about the arrow up or down..
9 ten thousandths is 0.0009, so it would be 9*10^-4