Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Q: Problem: if a - b = 6 and a 2 + b 2 = 50, find the value of ab.
Options: a. a. 7, b. b. 12, c. c. 15, d. d. 18, e. e. 20

A: a
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Q: Problem: if 5 a = 3125, then the value of 5 ( a - 3 ) is :
Options: a. 25, b. 125, c. 625, d. 1625, e. none of these

A: a
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Q: Problem: the points a ( 0, 0 ), b ( 0, 4 a - 5 ) and c ( 2 a + 1, 2 a + 5 ) form a triangle. if angle abc = 90, what is the area of triangle abc ?
Options: a. 80, b. 82.5, c. 78.5, d. 75.5, e. 74

A:
b
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