Answer: C. 10
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How to get this answer:
The largest angle ABD is 90 degrees because of the small little angle square marker. Another clue is the notation [tex]BA \perp BD[/tex]. The upside down T symbol means "perpendicular", showing the lines BA and BD form a 90 degree angle. The pieces of the angle (ABC = 4x and CBD = 5x) must add up to 90 degrees, or else they wouldn't fit like a jigsaw puzzle.
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(angle ABC) + (angle CBD) = angle ABD
4x + 5x = 90
9x = 90 ... combining like terms
9x/9 = 90/9 ... dividing both sides by 9
x = 10
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If x = 10, then 4x = 4*10 = 40, which is the measure of angle ABC
If x = 10, then 5x = 5*10 = 50 is the measure of angle CBD
We see that ABC+CBD = 40+50 = 90
