Hi,
While the tabular and algebraic approach are discussed, we can look at this question in another way.
So when ever 3 Type A are removed they are replaced with 4 of type B. So every year there will be 1 more aircraft in the fleet than previous year.
And for 60 planes to go to 50 % of its value which is 30 , it would take 10 years. But in 10 years the fleet size would be 60 +1* 10= 70 .
So definitely 30 is way less than 50% of the fleet
In 8 years Type A would be 24 less than today which is = 36 and Total fleet size would be 60+8= 68. But 50% of 68 is 34 but type a is more than 50 %
So in 9 Years it would have reduced by 27 which is 33, and fleet size would be 60+9= 69. 50%* 69= 34.5 So type A less than 50 %
hence D
Or You could set up an equation like this
No of planes in given year n = 60-3n+4n
Essentially We can say no of planes 1 year i.e 1980 = 61 and number of Type A = 60-3n =57 when n =1
So in 6,7,8,9,10 years it will be 66,67,68,69 70 receptively and number of type A aircraft would be 42,39,36,33,30 respectively.
So We see that by year 9 Type A aircraft are less than 50 % of total number of aircraft
probus
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Probus
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