<PRE><B>Question 259801</B>
10 + 50 = 80 - _______

Let d blank be x.. so it may looks like this

10 + 50 = 80 - x
80 -10 -50 = - x
80 - 60 = 20
20 = x
x = 20/1
x = 20

to check 
10 + 50 = 80 - 20
60 = 60


90 - 30 = 20 + ______
again let d blank be x
so it looks like below
90 - 30 = 20 + x
60 = 20 + x
Move 20 to the left side and it becomes -20
60 - 20 = x
40 = x

so to check whether your ans is correct 
90 - 30 = 20 + 40 (which is the x)
60 = 60
Juz remember to change the sign when you move it to the other side... + becomes minus and minuse becomes plus

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