Here,& p,pn∈{1,2,…50}
Now~p~can take values
2,3,5,7,11,13,17,23,29,31,37,41,43~and& 47.
∴~R1=(2,20),(2,21)…(2,25),(3,30),…(3,33),(5,50),…(5,52),(7,70),…(7,72),(11,110),(11,111),...(47,470),(47,471)
R2={(2,20),(2,21),(3,30),(3,31)....(47,470),(47,471)}
Now~R1−R2≡R1∩R2′
So~R1−R2={(2,22),(2,23),(2,24),(2,25),(3,32),(3,33),(5,52),(7,72)}
∴n(R1−R2)=8