Well, the rule is not easy to apply, but it helps: 6 euros equal 1000 ptas. So, 3 euros equal 500 ptas.
The 'rule of 6' is the most used by Spaniards. Fortunately we are slowly trying to think in euros, and forget the long gone pesetas.
Remember...
6 eur 1000 ptas
0.6 eur 100 ptas
0.3 eur 50 ptas
2000 ptas? easy, 2*6=12 eur
2500 ptas? well, 2*6+3=15 eur
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