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Aptitude Chain Rule Practice QA

25904.In a dairy farm, 40 cows eat 40 bags of husk in 40 days. In how many days one cow will eat one bag of husk?
1
$ \dfrac{1}{40} $
40
80
Explanation:

Let the required number of days be $ x $.

Less cows, More days (Indirect Proportion)

Less bags, Less days (Direct Proportion)

$\begin{cases}1 : 40 \\40 : 1 \end{cases}::40:x$

$\therefore$ 1 x 40 x $ x $ = 40 x 1 x 40

$\Rightarrow x $ = 40.

25906.If 7 spiders make 7 webs in 7 days, then 1 spider will make 1 web in how many days?
1
$ \dfrac{7}{2} $
7
49
Explanation:

Let the required number days be $ x $.

Less spiders, More days (Indirect Proportion)

Less webs, Less days (Direct Proportion)

$\begin{cases}1 : 7 \\7 : 1 \end{cases}::7:x$

$\therefore$ 1 x 7 x $ x $ = 7 x 1 x 7

$\Rightarrow x $ = 7.

25911.4 mat-weavers can weave 4 mats in 4 days. At the same rate, how many mats would be woven by 8 mat-weavers in 8 days?
4
8
12
16
Explanation:

Let the required number of bottles be $ x $.

More weavers, More mats (Direct Proportion)

More days, More mats (Direct Proportion)

$ \begin{cases}4 : 8 \\4 : 8 \end{cases}::4:x$

$\therefore$ 4 x 4 x $ x $ = 8 x 8 x 4

$\Rightarrow x $ =$ \dfrac{(8 \times 8 \times 4)}{(4 \times 4)} $

$\Rightarrow x $ = 16.

25913.A garrison had provisions for a certain number of days. After 10 days $\dfrac{1}{5}$ of the men desert and it is found that the provisions will now last just as long as before. How long was that?
50 days
30 days
40 days
60 days
Explanation:

Assume that initially garrison had provisions for $x$ men for $y$ days.

So, after 10 days, garrison had provisions for $x$ men for $(y-10)$ days

Also, after 10 days, garrison had provisions for $\dfrac{4x}{5}$ men for $y$ days $\left( ? x - \dfrac{x}{5} = \dfrac{4x}{5}\right)$

More men, Less days (Indirect Proportion)

(men) $x$ : $\dfrac{4x}{5}$ :: $y$ : $(y-10)$

$\Rightarrow x(y - 10) = \dfrac{4xy}{5}\\~\\$

$\Rightarrow (y - 10) = \dfrac{4y}{5}\\~\\$

$\Rightarrow 5(y - 10) = 4y\\~\\$

$\Rightarrow 5y - 50 = 4y \\~\\$

$\Rightarrow y = 50$

25915.If 8 men can reap 40 hectares in 12 days, then how many hectares can 30 men reap in 20 days?
175 hectares
225 hectares
250 hectares
275 hectares
Explanation:

40 : x : : $\begin{cases}8 &: 30 ---(Men) \\12 &: 2 ---(Hectares) \end{cases}$

8 × 12 × x = 30 × 20 × 40

x = $\dfrac{30\times 20 \times 40}{8 \times 12}$ = 250 Hectares

25916.18 men bind 900 books in 10 days. Find how many binders will be required to bind 600 books in 12 days?
10
11
13
15
Explanation:

We have to find the number of binders. Let the number of binders be x.

Direct Proportion:Less Books ($\downarrow$),Less binders($\downarrow$)

Indirect Proportion:More days ($\uparrow$),Less binders ($\downarrow$)

18 : x :: $\begin{cases}900 &: 600---(books) \\12 &: 10---(days) \end{cases}$

x × 900 × 12 = 18 × 600 × 10

x = $\dfrac{18\times 600 \times 10}{900 \times 12}$ = 10

44338.A man can walk a certain distance at a uniform speed in 100 days. How long will it take him to cover twice the distance at half the normal speed?
400 days
25 days
200 days
50 days
Explanation:

Earlier time = 100 days.

Distance is doubled and speed is reduced to half.

∴ time will become 2 × 2 i.e. 4 times.

Hence now it will take 100 × 4 = 400 days.

44339.A 100 m long 3 m high and 30 cm wide wall is built by 30 men, 20 women and 50 children working 9 hours a day in 20 days. How long a wall 1.5 m high 30 cm wide can be built by 15 men, 25 women and 35 children working 2 hour a day in 15 days (given men, women and children are equally efficient)?
75 m
25 m
50 m
100 m
Explanation:

Earlier dimensions of the wall = 100 × 3 × 0.30.
New dimensions = L × 1.5 × 0.3.
∴ As men, women and children are given to be equally efficient, so in the first case, the total number of persons is 100 (i.e. 30 + 20 + 50)
and the same in the second case is 75 (15 + 25 + 35).
Length of wall = L = (75x100) x (2x9) x (15x20) x (100 x 3 x 3 x 0.3)/(1.5 x 0.3) ⇒ L = 25 m.

44340. A certain number of men can finish a piece of work in 100 days. If there were 10 men less, it would take 10 days more for the work to be finished. How many men were there originally?
75
82
100
110
Explanation:

Originally let there be x men.
Less men, More days (Indirect Proportion) v Therefore, (x-10) : x :: 100 :110
=> (x - 10) * 110 = x * 100 => x= 110

44341. 2 men and 7 boys can do a piece of work in 14 days; 3 men and 8 boys can do the same in 11 days. Then, 8 men and 6 boys can do three times the amount of this work in
18 days
21 days
24 days
30 days
Explanation:

(2 x 14) men +(7 x 14) boys = (3 x 11) men + (8 x 11) boys
=>5 men= 10 boys => 1man= 2 boys
Therefore, (2 men+ 7 boys) = (2 x 2 +7) boys = 11 boys
( 8 men + 6 boys) = (8 x 2 +6) boys = 22 boys.
Let the required number of days be x.
More boys , Less days (Indirect proportion)
More work , More days (Direct proportion)
Boys22:11Work1 : 3}⋮⋮ 14:x
Therefore, (22 * 1 * x) = (11 * 3 * 14)
=> x = 21
Hence, the required number of days = 21

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