For the first 3 months of last year, the average daily rainfall in Lan

Question

For the first 3 months of last year, the average daily rainfall in Lancaster was 6cm. For last year as a whole, the average daily rainfall was 12cm. What was the average daily rainfall for the last 9 months of last year?

A. 14cm
B. 16cm
C. 18cm
D. 24cm
E. 30cm

A. 14cm
B. 16cm
C. 18cm
D. 24cm
E. 30cm

pushpitkc · Answer

It has been given that the average daily rainfall in Lancaster for the first 3 months was 6 cm.
Similarly, the average for the entire year is given to be 12 cm.

Let x be the average daily rainfall for each of the last 9 months.

Forming an equation,  \frac{3*6 + 9*x}{12} = 12 
Cross multiplying, 
 18 + 9x = 144 
 9x = 126 
 x = 14

Therefore, the average daily rainfall for the last 9 months of last year is  14(Option A)

generis · Answer

|__ A __|__ n __|__ S ___| 
 |__6__|__3__|__18__| 
 |_12__|_12__|__144_| 
 |_ 14 __|__9__|__ 126 _|

12 months' sum - 3 months' sum = 9 months' sum for daily average rainfall

144 - 18 =  126  cm total (sum) for 9 months

S/n = A

126 cm/9 months =  14  cm per month average daily rainfall

Answer A

JeffTargetTestPrep · Answer

Let’s assume that each month has exactly 30 days. For the first 3 months, there were 6 x 30 x 3 inches of rainfall, and for the entire year, there were 12 x 30 x 12 inches of rainfall.

Finally, for the last 9 months, the average daily rainfall was:

(12 x 30 x 12 - 6 x 30 x 3)/(30 x 9) = (12 x 12 - 6 x 3)/9 = 126/9 = 14 cm

Answer: A

stne · Answer

Hi JeffTargetTestPrep,

I was confused in this question too! It doesn't say MONTHY average rather it says DAILY average. Hence we need to know the number of days in each month to find the total rainfall, just as you have done .
But how can we assume that each month has 30 days .

If the question was " The monthly average for the first 3 months was 6 cm" then it would be ok to do 6 *3 to get the total rainfall of the first 3 months. Would it not ?
Please let me know what do you feel.
Thanks.

BrentGMATPrepNow · Answer

For the first 3 months of last year, the average daily rainfall in Lancaster was 6 cm.  
So, (sum of first 3 months)/3 = 6
Multiply both sides by 3 to get:  Sum of first 3 months = 18

For last year as a whole, the average daily rainfall was 12 cm   
So, (sum of all 12 months)/12 = 12
Multiply both sides by 12 to get: Sum of all 12 months = 144
We can write this as: ( sum of first 3 months ) + ( sum of last 9 months ) = 144
Rewrite as: ( 18 ) + ( sum of last 9 months ) = 144
Solve to get:  sum of last 9 months  =  126

So, average for last 9 months = ( sum of last months )/9
=  126 /9
= 14
= A

RELATED VIDEO FROM OUR COURSE
 https://www.youtube.com/watch?v=Y9f_0QJU1ug

Abhishek009 · Answer

So, 3*6 +9*x = 12*12

Or, 9x = 144 - 18

Or, x = 14

Thus, the answer must be (A) 14

testcracker · Answer

the easiest way you may like 12cm * 12 = 144cm 6cm * 3= 18cm so, (144 - 18) = 126cm rainfall in 9 month now, average daily rainfall for 9 month 126cm / 9 = 14cm thanks

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

For the first 3 months of last year, the average daily rainfall in Lan

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

For the first 3 months of last year, the average daily rainfall in Lan

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards