Solution:
Zero error of the Screw gauge e = 6 VSD - 5 MSD
Required diameter = e + R
=(6VSD−5MSD)+(32MSD−1VSD)=27MSD+5VSD=27mm+4.5mm=3.15cm
Alternate Approach:
10VSD=9MSD⇒1VSD=910MSD
Least count =1MSD-1VSD=(1-910)MSD
⇒0.1MSD=0.1×0.1=0.01 cm
As the zero of Vernier scale is before the main scale
Zero error =-(10-6)×( Least Count )=-4×0.01=-0.04 cm
Reading =3.1+1×( Least count )=3.1+1×0.01=3.11 cm
Therefore, true diameter = Reading - Zero error =3.11-(-0.04)=3.15 cm