According to the state regulations, the tax calculation method for individual publishing articles and books is as follows: 1. No tax shall be paid if the contribution fee is not higher than 800 yuan; 2. 14% of the contribution fee exceeding 800 yuan shall be paid if the contribution fee is higher than 800 yuan but not higher than 4000 yuan; 3. 11% of the whole contribution fee shall be paid if the contribution fee is higher than 4000 yuan; if someone has paid 420 yuan of tax in a lump sum, the tax shall be paid, How much is this person's contribution fee? If paid 770 yuan tax? It's a linear equation with one variable

According to the state regulations, the tax calculation method for individual publishing articles and books is as follows: 1. No tax shall be paid if the contribution fee is not higher than 800 yuan; 2. 14% of the contribution fee exceeding 800 yuan shall be paid if the contribution fee is higher than 800 yuan but not higher than 4000 yuan; 3. 11% of the whole contribution fee shall be paid if the contribution fee is higher than 4000 yuan; if someone has paid 420 yuan of tax in a lump sum, the tax shall be paid, How much is this person's contribution fee? If paid 770 yuan tax? It's a linear equation with one variable

If you pay 420 yuan, it means that his contribution fee is less than 4000420 / 0.14 + 800 = 3800. If you pay 770 yuan, it means that his contribution fee is more than 4000770 / 0.11 = 7000

A reservoir has an inlet pipe and an outlet pipe. It takes 5 hours to fill a whole pool with the inlet pipe alone, and 6 hours to drain a full pool with the outlet pipe alone. Now the two pipes are open at the same time, how long can it take to fill six fifths of the pool?

The water inlet pipe is 1 / 5 per hour, and the water outlet pipe is 1 / 6 per hour, so the two pipes are opened at the same time, and the water inflow per hour is 1 / 5-1 / 6 = 1 / 30, so the total time is 5 / 6 divided by 1 / 30, which should be 25 hours