z=(x-mean)/standard dev

so

z1=(13-12.5)/0.2

z2=(12-12.5)/0.2

look up z1 and z2 in the normal distribution tables.

Use these to find

P(x<z1)=P(x<13)

P(x<z2)=P(x<12)

Then P(x<13)-P(x<12) = P(12<x<13)

Multiply by 1000

Should work a treat.

-Do you get 68 wrong sized tubes?