Question

16
4. In city A's water purifying system, there are 3
levels of filtering. 30% of the impurity is filtered
out in the first stage itself and rest passes onto the
second screen. At the second level, 50% of the
incident impurities get obstructed while the third
stage blocks out 85% of the impurities reaching
this stage. Approximately, what percentage of the
water's impurities remain in city A's waters?
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Answer 1

Answer:

A. 2.9%

B. 3.5%

C. 5.25%

D. 7.25%

E. 12.75%

Answer 2

Answer:

The same can be done by successive percentages.

If you are using the equation where Overall % Change = a + b + ab/100, where a and b are the percentage changes, then the above equation can be used using 2 percentages at a time only.

Please Note: If a percentage is decreasing we take them as negative.

Since 30%, 50% and 85% are percentage decreases, first taking a = -30 and b = -50

-30 - 50 + (-30*-50)/100= -80 + 15 = -65

Now taking a = -65 and b = -85

-65 - 85 + (-65*-85)/100 = -150 + 55.25 = -94.75%. This means that 94.75% of the impurities are removed and therefore 100 - 94.75 = 5.25% still remains.

Option C