1. desktop x, leg 4x, equation:1/20 * x+1/400 * 4x =12, where x is a positive integer; (number of desktops =4* number of branches)

2.A x days, B 30-x days, equation: 500*x=250*(30-x), and x is a positive integer; (A and B parts are made into a set of products one by one, that is, A = B)

Third, eighth-grade students need xh to complete the rest alone. The equation is1/7.5 *1/5 *1/5 * x =1. The total time is t = x+65438. (Efficiency * Time = Workload)

4. There are X people at the beginning, and the equation is:1/80 * x * 2+1/80 * (x+5) * 8 = 3/4. (Personal efficiency * number of people * time = workload)