Question 1201030
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Building from what the tutor Edwin McCravy mentioned
x + y+z = 1000
x + <font color=red>y+z</font> = 1000
x + <font color=red>340</font> = 1000


This replacement, aka substitution, is valid because y+z = 340 was the second equation he formed.


Solving for x:
x+340 = 1000
x = 1000-340
x = 660
There were 660 people who responded "yes" (they find online travel reviews trustworthy).


We can then plug this into the third equation he formed to find y.
x-y = 400 
660-y = 400 
-y = 400-660
-y = -260
y = 260
There were 260 people who responded "no" (they don't find online travel reviews trustworthy).


Use that value of y to determine z.
y+z = 340
260+z = 340
z = 340-260
z = 80
There were 80 people who weren't sure if the online travel reviews were trustworthy or not.



Check:
x+y+z = 1000
660+260+80 = 1000
1000 = 1000 ..... 1st equation works
and
y+z = 340
260+80 = 340
340 = 340 ....... 2nd equation works
and
x-y = 400
660-260 = 400
400 = 400 ...... 3rd equation works
All three original equations are true for (x,y,z) = (660,260,80) 
Those values are fully confirmed.



Now to compute percentages, we divide each by 1000
x = 660 leads to x/1000 = 660/1000 = 0.66 = 66%
Therefore, 66% of the participants said "yes".
y = 260 leads to y/1000 = 260/1000 = 0.26 = 26%
26% of the participants said "no".
z = 80 leads to z/1000 = 80/1000 = 0.08 = 8%
8% of the participants said "not sure".



Summary:
x = 660 ----> 66% said "yes"
y = 260 ----> 26% said "no"
z = 80 -----> 8% said "not sure"
Note the percentages add to 100 percent ( 66+26+8 = 100)
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