Assuming that the total price is 20% profit, the cost of a commodity is:

[253-200× (1+20%)] (30%-20%) =130 (yuan)

(2) Original price of cantaloupe:

42 ÷ [(1-20%) × 3+(1-20%) × (1-20%) × 5] = 7.5 (yuan)

If all eight honeydew melons were bought on the third day:

7.5× (1-20% )× (1-20% )× (1-20% )× 8 = 30.72 (yuan)