Question 1190207
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Let's say there are 90 ball bearings in total. 
I picked something that's a multiple of 3, so that 1/3 of it is some whole number.


1/3 of this is (1/3)*90 = 30 ball bearings.


x = weight of the empty box
b = weight of 30 ball bearings
3b = weight of 90 ball bearings
All weight expressions are in pounds.


Based on the given info provided by your teacher, we can form these two equations
x+3b = 21
x+b = 9


Subtract the equations straight down
The x terms cancel and we are left with this equation
2b = 12
It solves to b = 6 after dividing both sides by 2.


We can then say,
x+b = 9
x+6 = 9
x = 9-6
x = 3
Or you could say
x+3b = 21
x+3*6 = 21
x+18 = 21
x = 21-18
x = 3



Answer: B) 3 pounds
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