well here is how i go about it,

first of all i basically try to figure out how much percent of the total work can be done by A/B/C (etc) per time unit specified (hrs/days/weeks etc.)

so if we get this info, A can complete painting a wall in 12 days, then % of wall he can paint in one day would be 8.5%

and also if B can complete painting a wall in 20 days, then % of wall B can paint in one day would be 5%.

Now with this info we can do any combination in seconds

.

like A paints for 2 days, then B joins in. How much more time will it take them now?

in 2 days A paints 8.5 x 2 = 17%.

A and B together paint 8.5 + 5 % of the wall per day = 13.5% of the wall per day

So % of wall left is 83%, per day can be painted 13.5% . no of days we need = 83/13.5 which will be close to one of the options you have. (very bad numbers to take for an example

)

you can apply this to any kind of work related probs. I prefer dealing with %ages rather than fractions.

Just ask urself what can be done faster :- (1/15 + 3/5 + 1/20) or (6.66% + 60% + 5%)?

Thats just my approach, i know there are better ones .

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The Legion dies, it does not surrender.