Aptitude - Time and Work - Discussion
Discussion Forum : Time and Work - General Questions (Q.No. 1)
1.
A can do a work in 15 days and B in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is :
Answer: Option
Explanation:
| A's 1 day's work = | 1 | ; |
| 15 |
| B's 1 day's work = | 1 | ; |
| 20 |
| (A + B)'s 1 day's work = |  |
1 | + | 1 |  |
= | 7 | . |
| 15 | 20 | 60 |
| (A + B)'s 4 day's work = | |
7 | x 4 | |
= | 7 | . |
| 60 | 15 |
| Therefore, Remaining work = | |
1 - | 7 | |
= | 8 | . |
| 15 | 15 |
Discussion:
344 comments Page 15 of 35.
Merlin said:
1 decade ago
Can you just explain why they are subtracting the answer from 1?
Merlin said:
1 decade ago
Why do we make an assumption that the total work is 1?
Fuhar choudhury said:
1 decade ago
I don't understand the logic behind solving such problems. Like people can assume anything anywhere I mean in the first problem itself you assumed let the amount of total work be one but why can't we take 2, 3, 4,..1001, etc. This is the reason mental maths eats me up mentally.
Mangesh said:
1 decade ago
When they not mention the quantity of work in problem in that case assume work done is 1. For e.g A can manufacture 1 chair in 15 days while B takes 20 days to manufacture 1 chair in this work done is only 1 that is to manufacture a chair but days require is different.
Akash said:
1 decade ago
Try to avoid fraction calculation.
L.C.M of(15,20) = 60.
Let total work is 60 unit.
A can do 60/15 = 4 unit/day.
B can do 60/20 = 3 unit/day.
A+B can do = 4+3 = 7 unit/day.
A+B in 4 day can do = 7*4 = 28 unit.
Total work = 60 unit.
Remaining work = 60-28 = 32 unit.
Fraction of work = (Remaining work/Total work) = 32/60 = 8/15 (ans).
L.C.M of(15,20) = 60.
Let total work is 60 unit.
A can do 60/15 = 4 unit/day.
B can do 60/20 = 3 unit/day.
A+B can do = 4+3 = 7 unit/day.
A+B in 4 day can do = 7*4 = 28 unit.
Total work = 60 unit.
Remaining work = 60-28 = 32 unit.
Fraction of work = (Remaining work/Total work) = 32/60 = 8/15 (ans).
/mat said:
1 decade ago
Why 1/15, 1/20?
Devi sri prasad said:
1 decade ago
I think LCM is the best solution.
Avinash Ghadge said:
1 decade ago
@Deppa.
1/15 & 1/20 hear denominator are not same ok. So firstly we have to make both same. So 1/15 are multiply bye 4 on both numerator, denominator we get:
1*4/15*4 = 4/60.
1*3/20*3 = 3/60 this value are getting now adding this two value, we get.
4+3/60 = 7/60.
That's it. I hope you understand.
1/15 & 1/20 hear denominator are not same ok. So firstly we have to make both same. So 1/15 are multiply bye 4 on both numerator, denominator we get:
1*4/15*4 = 4/60.
1*3/20*3 = 3/60 this value are getting now adding this two value, we get.
4+3/60 = 7/60.
That's it. I hope you understand.
PriyA said:
1 decade ago
But still I can't understand the logic behind the subtraction of 1. I have read @Mangesh answer. But why do we assume "one"? They did't mention anything right?
Priya said:
1 decade ago
Can any one please say, how (1-7/15) is applied?
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