2. If x cubic meters is used as a table top, (12-X) square meters is used as a table foot.

4×20X=400( 12-X)，

X= 10， 12-X=2，

3. if it takes x days to manufacture part a, it takes (30-X) days to manufacture part b,

500X=250(30-X)，

X= 10，30-X=20，

4. Setting * * * takes x hours.

1/7.5+X/5= 1，2/ 15+X/5= 1，

2+3X= 15

X= 13/3 hours,

5. Let's assume that X people arrange 2 hours.

2X/80+8(X+5)/80=3/4，

X=2 .