question: If X is the sum of first 10 positive even integers and Y is the sum of first 10 positive odd integers, what is the value of x-y?
Please try to explain your answers
Options:
(A) 10
(B) 25
(C) 50
(D) 75
(E) 100
answer: Stream of consciousness: First even minus first odd = 2-1 = 1;
The sum of first 2 even integers minus the sum of first 2 odd integers = (2+4)-(1+3) = 2;
The sum of first 3 even integers minus the sum of first 3 odd integers = (2+4+6)-(1+3+5) = 3;
We can see the patterns here, so the sum of first 10 positive even integers minus the sum of first 10 positive odd integers will be 10. The answer is (A).
--
question: It is the New Year and Mandy has made a resolution to lose weight this year. She plans to exercise and do yoga. For exercise she plans to workout at the gym and ride her bicycle in the ratio of 2:3 everyday. She will also do yoga in the ratio, yoga:exercise = 2:3. If she rides her bike for 20 minutes, how much time will she spend doing yoga? (rounded to minutes)
Options:
(A) 10 min.
(B) 33 min.
(C) 17 min.
(D) 23 min.
(E) 25 min.
answer: Stream of consciousness: The ratio is 2:3 = gym:ride, so (20)(3/2) = 30 minutes at the gym, and 30 + 20 = 50 minutes exercise, so (2/3)(50) = 33 minutes yoga. The answer is (B).
--
question: The average of 11 results is 46, if the average of first six results is 49 and that of the last six is 52. Find the sixth result?
Options:
(A) 21
(B) 56
(C) 100
(D) 25
(E) 23
answer: Stream of consciousness: 1 to 11 = 11 * 46 = 506
1 to 6 = 6 * 49 = 294
6 to 11 = 6 * 52 = 312
6th = 294 + 312 506 = 100 The answer is (C).
--
question: For a particular American football game, the probability of a team's quarterback throwing a completed pass on each throw is 3/10. What is the least number of times that the quarterback should throw the ball that will increase the probability of getting a completed pass at least once to more than 50%.
Options:
(A) 10
(B) 5
(C) 2
(D) 3
(E) 1
answer:
Stream of consciousness: Rule of Subtraction: P(A) = 1 - P(A')
Rule of Multiplication: P(A B) = P(A) P(B)
The probability that the quarterback throws a completed pass at least once in 2 throws is 1- (7/10)^2 = 1 - 49/100 = 51/100 > 50% The answer is (C).
--