alanforde800Maximus wrote:Ques) What is the average of a, b, and c?
(1) The average of a and b is c.
(2) The average of b and c is 4.
Target question: What is the average of a, b, and c?
NOTE: since the average = (a+b+c)/3, our goal here is to find the SUM a + b + c
So, we can REPHRASE the target question....
REPHRASED target question: What is the value of a + b + c?
Statement 1: The average of a and b is c
This means that (a + b)/2 = c
Multiply both sides by 2 to get: a + b = 2c
There are several values of a, b, and c that satisfy statement 1. Here are two:
Case a: a = 1, b = 1 and c = 2, in which case
a + b + c = 1 + 2 + 2 = 5
Case b: a = 1, b = 2 and c = 3, in which case
a + b + c = 1 + 2 + 3 = 6
Since we cannot answer the
REPHRASED target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: The average of b and c is 4
Since there's no information about a,
there's no way to find the value of a + b + c
Since we cannot answer the
REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined
Statement 1 tells us that
a + b = 2c
Statement 2 tells us that (b + c)/2 = 4, which means
b + c = 8
So, we have the following system:
b + c = 8
a + b = 2c
IMPORTANT: Many students will look at this system and
incorrectly conclude that, since we have 2 equations and 3 variables, there's no way to answer the target question. This is not necessarily the case, because the target question does NOT ask us to find the
individual values of a, b and c. The target question asks us to find the SUM of a, b, and c. So, for example, if the two equations happened to be
a + b + 2c = 5 and
2a + 2b + c = 19, then we could ADD the equations to get
3a + 3b + 3c = 24, and upon dividing both sides by 3, we'd see that
a + b + c = 8, in which case we COULD answer the target question despite only having 2 equations with 3 variables.
So, we can't just automatically assume that we don't have sufficient information.
So, let's TEST some values of a, b and c that satisfy BOTH equations.
NOTE: since the equation
b + c = 8 doesn't have the variable a, let's use it to first find some values of c and b, then we'll use the other equation,
a + b = 2c, to find the corresponding a value.
Case a: b = 1 and c = 7 satisfies the equation
b + c = 8 . Now plug these values into the other equation (
a + b = 2c) to get: a + 1 = 2(7). Solve to get: a = 13.
So, the values are a = 13, b = 1 and c = 7, which means
a + b + c = 13 + 1 + 7 = 21
Case b: b = 0 and c = 8 satisfies the equation
b + c = 8 . Now plug these values into the other equation (
a + b = 2c) to get: a + 0 = 2(8). Solve to get: a = 16.
So, the values are a = 16, b = 0 and c = 8, which means
a + b + c = 16 + 0 + 8 = 24
Since we cannot answer the
REPHRASED target question with certainty, the combined statements are NOT SUFFICIENT
Answer =
E
NOTE: Here's a video with tips on
rephrasing the target question:
https://www.gmatprepnow.com/module/gmat- ... cy?id=1100
