2, (90/5)*30=540 (tree)

3,500-20-65 * 4/4 = 55 (km)

4.1500-(1500-800) * 20% =1360 (yuan)

5、 24/? *( 1-37.5%)-24=

6. Suppose that X is needed to complete the work, and B can manufacture Y parts per hour.

18 * x:y * x = 3:5； Y=30 can be calculated. That is, B can make 30 parts per hour.

30 *12 * 3/(3+5) =135 (pieces)