Question 826893: Which option is correct?
5 , ? , 4 , 7.5 , 17 , 45
A. 3.5
B. 3
C. 2.5
D. 2
Answer by Edwin McCravy(20056)   (Show Source):
You can put this solution on YOUR website!
5, ?, 4, 7.5, 17, 45
Let x be the missing number
5, x, 4, 7.5, 17, 45
List all 6 terms in a column
5
x
4
7.5
17
45
Subtract each number or expression from the one immediately
underneath it, write it out beside that number in a new column
and simplify, if necessary
5 x-5
x 4-x
4 7.5-4=3.5
7.5 17-7.5=9.5
17 45-17=28
45
Make a third column the same way
5 x-5 (4-x)-(x-5)=9-2x
x 4-x 3.5-(4-x)=x-.5
4 7.5-4=3.5 9.5-3.5=6
7.5 17-7.5=9.5 28-9.5=18.5
17 45-17=28
45
Make more columns the same way. Continue until there are just two
in the last column
5 x-5 (4-x)-(x-5)=9-2x (x-.5)-(9-2x)=3x-9.5 (6.5-x)-(3x-9.5)=16-4x
x 4-x 3.5-(4-x)=x-.5 6-(x-.5)=6.5-x 12.5-(6.5-x)=6+x
4 7.5-4=3.5 9.5-3.5=6 18.5-6=12.5
7.5 17-7.5=9.5 28-9.5=18.5
17 45-17=28
45
Set the two expressions in the last column equal to each other:
16-4x = 6+x
10 = 5x
2 = x
The missing number is 2.
Edwin
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