(1) A book with 80 pages. Look at its 1\4 the first time, and look at the remaining 2\3 the second time. How many pages are left?

I saw it the next day: [1-1/4] * 2/3 =1/2.

Also: 80 * [1-1/4-1/2] = 20 pages.

(2) For a project to build roads, 5\27 of the total length was built on the first day, 3\ 1 1 on the second day, 5\6 on the third day and 8 km on the fourth day, which has just been completed. Find out the length of this road.

I did it the next day: [1-5/27] * 3/11= 2/9.

I did it on the third day: [1-5/27-2/9] * 5/6 = 40/81.

I did it on the fourth day:1-5/27-2/9-40/81= 8/81.

Then the total length is: 8/[8/8 1]=8 1 km.

(3) How many kilograms of water does it take to dilute 30 kilograms of salt water with 15% into salt water with a salt content of 5%?

The amount of salt is 30* 15%=4. 5 kg

Now the total amount of salt water is: 4. 5/5%=90 kg

To add water: 90-30 = 60kg.