[Task] Where did 10 rubles go?

This task explodes my brain. It seems that I have tried to solve it before, but to no avail. A short search on the network did not return any results.

One person asks a friend to lend. He gives him 100 p. and he puts them in his pocket. Then this person meets the second friend and also asks for a loan. He gives him 50 p. and the person puts them in another pocket, walks into the store and buys a chocolate bar for 20 rubles. At the same time he pays with a 50-ruble note, which the second friend gave him, and receives a change of 30 rubles. And then he decided to return to the first friend at least some part of the debt. He meets him and gives him 30 rubles, which he received as change after purchasing a chocolate bar.

After that, he decided to calculate the total balance. He owed his first friend 70 rubles. The second - 50 p. And he also has a chocolate bar for 20 rubles. In the amount of 140 p. He occupied in general 150 r.
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Where are the 10 p.

PS: Got real money, put it in piles - it all fits. But why in theory such an imbalance I do not understand ...