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To satisfy your curiosity regarding the term "Time", see this article from Wikipedia.
It starts with the words
"Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession
from the past through the present to the future".
But do not be scary !!! Usually just the children of the kindergarten level know enough about this notion/conception.
As I understand, you have difficulties understanding joint work problem solutions.
Believe me, you do not need to know more about these terms than you just know for now.
Simply look into the lessons
- Using Fractions to solve word problems on joint work
- Solving more complicated word problems on joint work
- Selected joint-work word problems from the archive
Read them and get be trained in solving joint-work problems.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic "Rate of work and joint work problems" of the section "Word problems".
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Time is too fundamental conception to define it using words.
Accept it as is.
(The common sense is that what the people gain to the age 18, when they forget all they learned in the school ! - Famous definition by Einstein . . . )
The rate of work is the amount of work (a part of an entire/whole work) performed at the init of time (day/hour/minute . . . ).
It measures in units .