$3$ sons that must sell in the same price

Question

A father send his $3$ sons to sell watermelons.The first son took $10$ watermelons,the second $20$ and the third $30$ watermelons.The father gave order to sell all in the same price and collect and the same number of money.How is this possible?

Any ideas for this puzzle?

Answer 1

We can model this as: $$ 10 p = 20 p = 30 p \Rightarrow $$ Subtracting $10 p$ from both sides of the first equation gives $$ 0 = 10 p \Rightarrow \\ p = 0 $$

Answer 2

Son 3 sells 30 watermelons, then buys 20 watermelons from Son 2. Son 2 buys 10 watermelons from Son 1.

Answer 3

Son 3 sells 20 watermelons then gives 10 watermelons to son 1. Son 2 and son 1 then sell 20 watermelons each.