Hi basky we can attack Time and Work problems in 2 ways...One of the golden rule already vidya has taken care. 2nd is some problems can also be done with proportion method which is very easy.... by practice u can know which one should be applied. usually when problems ask together how much or alone how much can a person do i apply above rule.
2nd , Time taken to finish a work is inversely proportional to the number of persons working at it.
(more persons, less it the time taken to finish a job)
example: if 16 men working 7 hours a day can plough a field in 48 days, in how many days will 14 men woking 12 hours a day plough the same field.
what u have to find out keep on right. in this case we have to find days. so days are on right
days
men 16 : 14 :: 48 : x // now compare men with days...less men(14) compared to previous (16) ,..so more days....so indirect proportion
hours 7 : 12 // same thing more hrs less days (indirect)
now the proportion is
14 :16 : : 48 : x
12 : 7
16 x 7 x 48
x=------------ = 32 days
14 x 12
if it is indirect , no.s get suffled
hope it help
good luck
anu