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8 votes
8 votes

The population of a new city is $5$ million and is growing at $20\%$ annually. How many years would it take to double at this growth rate? 

  1. $3-4$ years
  2. $4-5$ years 
  3. $5-6$ years 
  4. $6-7$ years

6 Answers

0 votes
0 votes

Option : A

rule of 72(when amount doubled in compound interest annually): 72/rate = time

therefore, time= 72/20=3.6 years

i.e, 3 - 4 years

NOTE: this formula gives approximate answer

Similarly when amount tripled in C.I then time=144/rate

–1 votes
–1 votes
When population 100 then 20 men increase in 1 year

  "         "             1    "      20  "       "          " 1⨉100  years

  "         "             1    "       1  "       "          " 1⨉100 /20 years

  "         "         5000000   "   1  "       "        "   1⨉100  /(20⨉5000000)

  "         "         5000000  "   5000000 "   "    "   1⨉100 ⨉5000000 /(20⨉5000000) = 5 years

Ans will (C) [As min 5 years needed for doubling growth rate]
Answer:

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