SAT Math Multiple Choice Practice Question 145: Answer and Explanation

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Question: 145

13. 25% of the marbles in a jar are red. After 20 red marbles are added to the jar, 50% of the marbles are now red. How many total marbles were originally in the jar?

A. 12
B. 16
C. 20
D. 40
E. 80

Correct Answer: D

Explanation:

D. This question is an algebra problem. Let n equal the number of marbles in the jar. Therefore, originally 0.25n (or ) of the marbles are red. When you add 20 red marbles, 0.25n + 20 are now red, out of n + 20 total. Because that's 50%, you can write 0.25n + 20 = 0.50(n + 20), which gives you 0.25n + 20 = 0.50n + 10. Combining like terms gives you 10 = 0.25n, and multiplying both sides by 4 (or dividing by 0.25) gives you n = 40.

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