The manual » Extra

1. Unlock countries

For example, Spain and Poland

If you play all countries in Europe without completing the bonus levels, you will only gain access to Germany, the United Kingdom, France, and Italy. To unlock countries, you must complete all bonus levels for a country. The combinations of countries for which you complete all bonus levels determine which countries you unlock.

Take Germany and the United Kingdom, for example. Germany deals with addition and the United Kingdom deals with subtraction. If you complete all the bonus levels in both countries, you unlock Spain. In Spain, you are given sums that require you to add and subtract, such as 7 + 5 - 3.

In France, you will be multiplying. If you complete all the bonus levels in Germany and France, you will be given sums involving addition and multiplication. For example, 3 x 5 - 2.

Eventually, you will encounter sums in which you will use all four operators. This will result in sums such as 7 x 5 + 3 - 6 : 2.