The chart shows the water level in a reservoir over time.

Question

https://gmatclub.com/forum/download/file.php?id=86248 The chart shows the water level in a reservoir over time. The vertical axis represents the water level, in meters, relative to the bottom of the reservoir. The horizontal axis represents the time, in minutes, since monitoring began. The water level generally decreases as water is used for irrigation, but occasional rainfall causes the level to rise temporarily. Select the option from each drop-down menu that creates the statement that most accurately reflects the information provided. If the average rate of water level decrease monitored over the seven hours continues, the water level would reach zero after GRAPH_1_PLACEHOLDER minutes. The largest decrease in water level during any 30-minute interval shown on the graph was GRAPH_2_PLACEHOLDER the largest increase during any 30-minute interval. GMAT-Club-Forum-64uxw4e0.png GMAT-Club-Forum-3tqqpjsn.png

Bunuel · Accepted Answer

Drop-down 1:

Over 420 minutes, the water level dropped from 120 meters to 30 meters, a total decrease of 90 meters. At the same rate, the remaining 30 meters, which is one-third of 90, would take one-third of the time, so 420/3 =  140 minutes .

Drop-down 2:

From the graph, the steepest 30-minute decrease occurred between 120 and 150 minutes, when the level fell from 105 meters to 75 meters (a 30-meter drop). The steepest 30-minute increase occurred between 300 and 330 minutes, when the level rose from 75 meters to 90 meters (a 15-meter rise). The largest decrease (30 meters) is exactly  twice as much  as the largest increase (15 meters).

I29-164

f3rn4nd44m0r1m · Answer

First, lets calculate the average rate of water decrease over the seven hours (420min). rate = work / time, in this case, rate=(120-30)/420=3/14
Than, lets calculate how many time we need to bring the level from 30 to 0 with this rate. 
If rate = work/time, time=work/rate, so t=30/(3/14)=  140minutes

The largest decrease is from the 360 to 390 minutes with arround 33m decrese. 
The largest increase is from the 300 to 330 minutes with arround 15m incrase.
Hence, the largest decrease  is twice as much  as the largest increase

Dereno · Answer

The left point of the graph is at 120 metres and the right side is at 30 metres. So the decrease is (120-30)= 90 metres.

Using unitary method

90 metres needs 420 minutes , we need a drop of 30 metres to reach zero.

30 metres needs x minutes.

X = (420*30)/90

X =  140 minutes.

Question 2:

The largest decrease and largest increase during a 30 minute interval are :

Largest decrease = (105-75) = 30 metres. ( 120 - 150) minutes.

Largest Increase = (75 - 90) = 15 metres at (300 - 330) minutes.

30 = ————- * 15

30 = Twice * 15

Twice .

findingmyself · Answer

We essentially have to calculate hourly average drop. For the same, let's first calculate individual drop numbers on a hourly basis. For eg: From 120 to 100 in first 60 minutes, the drop is -20
Thus we get the following values: -20, +5, -25, -10, +5, 0 and -40. Total hours is 7 hours
Average is sum of (-20+5-25-10+5+0-49)/7
This comes out to be approximately 12-13 meters.
  Thus for 30 meters, rougly 2 and half hours or close to 140 minutes  .

The largest decrease in water level during any 30-minute interval shown on the graph was the largest increase during any 30-minute interval.
Looking at larges increase which is from 75 to 105 which happens in 120 minutes to 150 minutes contributing  40%  \frac{(105-75)}{75} *100=40%
Now the maximum increase have happened from 75 to 90 meters in 330 minutes which is 20% increase. Hence twice as much.

vxdhi · Answer

Hi, Isn't the largest decrease actually at min.360 when it falls from 75 to approx 40 in 30 mins?

Thanks

Bunuel · Answer

Replaced the graph. Thank you!

The chart shows the water level in a reservoir over time.

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