Assuming simple interest, with a starting amount (principal) of A, interest I after 10 years =10A(r/100)=Ar/10 where r is the percentage rate. After 10 years the total amount is A+I=A+Ar/10=3A, because after 10 years A has grown to 3A.
So we need to solve A+Ar/10=3A for r. We can divide through by A:
1+r/10=3, r/10=2, r=20%.