SI recruitment exam: Manage your calculation time for this section

By Banda Ravipal Reddy Director, SIGMA Sai Institute of General Mental Ability Hyderabad This is in continuation to the last article on preparation for the Sub-Inspector of Police recruitment exam. Here are some practice questions and explanations on the Time and Work topic. To improve calculations speed, the aspirants must practice questions from the previous […]

By Banda Ravipal Reddy
Director, SIGMA
Sai Institute of General Mental Ability
Hyderabad

This is in continuation to the last article on preparation for the Sub-Inspector of Police recruitment exam. Here are some practice questions and explanations on the Time and Work topic. To improve calculations speed, the aspirants must practice questions from the previous question papers.

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1. X and Y can do a piece of work in 10 days and 12 days respectively. If they work alternate days starting with X, the work will be finished in ___ days.

A. 10 1/6 B. 10 5/6 C. 10 7/6 D. 10 5/9

Ans:B
Explanation:
(A B)’s 2 day work is= 1/10 1/12= 6 5/60 =11/60.
Work done in 5 pairs of days= 5× 11/60.
Remaining work=1- 55/60= 5/60

On 11th day it’s A turn,

∴ 5/60 Work is done by A=10× 5/60 = 5/6 days.

Total time taken=10 5/6=10 5/6 days.

2. A started a work and left after 2 days. Then B finished the work in 9 days. Had A left the work after working for 3 days, B would have finished the work in 6 days. Each of them working alone finish the whole work in ____ days.

A. 5, 15 B. 15, 5 C. 6, 9 D. 9, 6

Ans: A

Explanation:
Let A finish the work in X days & B finish the work in Y days

2/x 9/y =1 & 3/x 6/y=1
∴by solving, 3/x-2/y= 6/y-9/y
Y=3X
By substituting Y value, will get X=5 & Y=15.

3. A, B and C can do a piece of work in 16 days, 24,days and 32 days viz. They started the work together but A left after 4 days and B left 3 days before the completion. The work will be completed in ___ days

A. 10 B. 12 C. 8 D. 7

Ans:B
Explanation:
(A B C)’s 1day work = 1/16 1/24 1/32 = 6 4 3/96 = 13/96

(A B C)’s 4 day work = 4× 13/96 = 52/96

Work done by C in 3 days = 1/32 ×3 = 3/32

. Remaining work =1-( 52/96 3/32)
=1-(52 9/96 ) = 35/96

B and C can do 7 units of work per a day

∴ 35 units work can do in= 35/7 = 5 days.
The work will be completed in =4 3 5=12 days.

4. A takes as much time as B and C together take to do the work. A and B together can do it in 20 days. If C alone can do in 30 days, B alone can do it in _____ days.

A. 100 B. 110 C. 120 D. 130

Ans: C

Explanation:
Given, A=B C

C = 1/30
(A B)’s 1 day work= 1/20
A B C’s 1 day work= 1/20 1/30 = 3 2/60 = 5/12

B C B C= 5/12 {since, A=B C}

2(B C)= 5/12 =>;B C= 5/24

B= 5/24 – C = 5/24 – 1/30 = 5-4/120 = 1/ 120

∴B alone can do it in 120 days.

5. To do a job B takes three times as long as (A C) can do together and C twice as long as (A B) can do together. If the three men together complete the work in 50 days, each takes separately for___ days.

A. 120-200-150 B. 120-150-200

C. 120-200-180 D. 120-150-300

Ans: A

Explanation:
Given, 3B=A C;

2C=A B

A B C= 1/50
3B B= 1/50 (since,3B=A C), B= 1/200

∴B did work in 200 days.

2C C= 1/50 =>;C= 1/150
C did work in 150 days.

From the above equation, A= 1/50 -(1/200 1/150 ) = 12-7/600 = 1/120
∴A did work in 120 days