P40,000 would be the correct answer if you use "simple interest" in the computation. But thanks to compound interest, your money will actually be P51,874 after 10 years. The total interest earned is more than P31,874 instead of just P20,000. This is because the interest earned each year will also earn interest in the succeeding years.
Compound Interest + Time
Before I forget, the table below shows how the original P20,000 principal grows each year. Notice that the interest earned every year is increasing and not fixed at P2,000. So let's start investing now! Don't procrastinate!