1.5' s child has 23 pieces of candy. Five dollars for each child and three dollars for the fifth child.

For six children, there are 26 pieces of candy. Each person gives 5 yuan, and the sixth child gives 1 yuan.

So, there are at least five children.

This problem can be calculated by equation, or you can think so. There are 8 pieces of sugar left, and each child will increase from 3 pieces to 5 pieces. Everyone takes 2 pieces of candy from the rest and 8 pieces of candy for 4 children. The fifth child will have no sugar to eat, only the original three pieces, so there will be at least five children.

2.2+4+8+16+32+64 =126 (person)

Answer; By the sixth week, the number of clubs was 126.

3. Solution: Let A complete X meters and B complete Y meters.

1/4X+ 1/5Y-40 = 780； X+Y=3600

Solution:

X=2000

Y= 1600

A: A is 2000 meters, and B 1600 meters.

I hope you can be satisfied with my answer! thank you