10 1/2 of what number is 42.

10 1/2 = ( 20 / 2 ) +( 1 / 2 ) = 21 /2

42 = 84 / 2

42 / ( 10 1/2) = ( 84 / 2 ) / ( 21 / 2 ) =

84 / 21 = 4