6a single sheet of aluminium foil is folded twice to produce a stack of four sheets. the total
thickness of the stack of sheets is measured to be (0.80 ± 0.02) mm. this measurement is made
using a digital caliper with a zero error of (−0.20 ± 0.02) mm.
what is the percentage uncertainty in the calculated thickness of a single sheet?

a 1.0% b 2.0% c 4.0% d 6.7%

The correct answer is option C. i.e.  4%

The total thickness of the stack of sheets is measured to be (0.80 ± 0.02) mm. 

The maximum true thickness will be (0.80 + 0.02) mm. = 1.00 mm
Ans, the maximum uncertainity will be  (0.20 + 0.02) mm = +/-0.04 mm

Thus, the four sheets true thickness will be 1.00 mm +/-0.04 mm
Thus, single sheet thickness will be 0.250 mm +/-0.01 mm
Thus, percentage error = (0.01/0.25) * 100 = 4%

26.4 is about the right time