Use the accompanying data to estimate with a 95% confidence interval the difference between true average compressive strength (N/mm2) for 7-day-old concrete specimens and true average strength for 28-dayold specimens

1.  Straight lines are 180 degrees.  That means 180-126=x.  So, x=54.  

You can then use x to find y.  180-54=y.  y=126.  OR  You can use vertical angles to find y.  Vertical angles are the same degree.  So if y is vertical to the angle with 126 degrees, then y=126.

Either way, you will get x=54 and y=126.

2.  You would solve problem 2 the same way as problem 1.  I encourage you to try to do it yourself, but here's how you would do it.

180-103=y, so y=77.  You can either use vertical angles or x=180-77 to find x.  You will end up with x=103.

So y=77 and x=103.

3.  Here you would use vertical angles.  Remember, vertical angles have the same degree!  So 2x+29=5x+20.  Then all you have to do is solve it.

You would end up with x=3.

I hope that helps!