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? $3-4$ years $4-5$ years $5-6$ years $6-7$ years Quantitative Aptitude gate2014-ag quantitative-aptitude simple-compound-interest normal + – makhdoom ghaya asked Mar 21, 2016 makhdoom ghaya 7.0k views answer comment Share Follow See all 3 Comments See all 3 3 Comments reply air1 commented Jan 14, 2017 reply Follow Share This can also be solved very easily by using the rule of 72 (http://www.investopedia.com/ask/answers/04/040104.asp) the time taken by a quantity to get doubled at a constant growth rate is $72/rate$. so for this case answer is $72/20 = 3.6$. 7 votes 7 votes Chhotu commented Nov 18, 2017 reply Follow Share Can we do it via Compound interest formula ? 1 votes 1 votes Kiyoshi commented Nov 27, 2021 reply Follow Share Obviously Yes @Chhotu.. 1 votes 1 votes Please log in or register to add a comment.
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 Tez answered Jan 21 Tez comment Share Follow See all 0 reply Please log in or register to add a comment.
–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] srestha answered Mar 21, 2016 srestha comment Share Follow See 1 comment See all 1 1 comment reply Arjun commented Mar 21, 2016 reply Follow Share "growing at 20% annually" means every year the population increases by 20% or becomes 1.2 times that at the start of the year. In your approach you have considered it to be 20% every year increase, from the initial value at start of year 1. 2 votes 2 votes Please log in or register to add a comment.