Where S.I = Simple Interest P= Principal amount (Amount invested or loan taken\/Initial value) R= Rate T= Time (years) You can memorize all the formula or memorize only the main one and learn how to make the rest the subject R=(S.I×100)∕(P×T) S.I=45 P=450 T=4 ⇒R=(45×100)∕(450×4)=5∕2=2.5% NOTE: 45 goes into 450, 10 times and 10 goes into 100, 10 times and 2 goes into 10, 5 times and 4, 2 times. Then divide 5 by 2