If 1 / a = B + 3 / a = 1 / 7 (a, B are natural numbers), then B=

If 1 / a = B + 3 / a = 1 / 7 (a, B are natural numbers), then B=

∵1/a=1/7
∴a=7
b+7/3=1/7
b=-46/21

If a x one seventh + B x one seventh = 30, then 2 (a + b) = ()
1 1
If a X - + B x - = 30, then 2 (a + b) = ()
7 7

a×1/7+b×1/7=30
Extract 1 / 7 to get
1/7×（a+b）=30
a+b=210
∴2（a+b)=2×210=420

As soon as you see the work, Party A and Party B will cooperate for 12 days and Party A will do it alone for 21 days. How many days will Party B finish it alone

Let B do it alone for X days
1/21+1/x=1/12
That is to say, a's daily efficiency plus B's daily efficiency is 1 / 12
The solution is x = 28

A job is completed in 12 days by Party A and Party B, 21 days by Party A alone, and how many days by Party B alone?

If a does it alone for 21 days, then a does 1 / 21 every day
The cooperation between Party A and Party B is completed in 12 days, and the cooperation between Party A and Party B is completed in 1 / 12 every day
So B does 1 / 12-1 / 21 every day = 1 / 28
Then B alone needs 28 days
If you can, give me a good comment