Sherlock Holmes’ mysterious 221
221B Baker Street happens to be the house of Sherlock Holmes. Incidentally, the number 221 has several significances. 221 = 13×17 is a product of two prime numbers. It is called a semi-prime number. Some other examples are 6(2×3), 10(2×5), 14(2×7), 21(3×7), 22(2×11), 26(2×13), 33(3×11), 35(5×7), 39(3×13), 49(7×7), 51(3×17), 55(5×11), 57(3×19), 62(2×31), 65(5×13), 69(3×23), 77(7×11),…