The mass of an object is reported as 5.20 g +0.05g. 0.05 g indicates the

I am thinking there is an error in or sign. it is supposed to be  ± instead of st + as in 5.20 g  ± 0.05g.

0.05 g indicates the Measurement of Uncertainty

The mass of an object is reported as 5.20 g  ±0.05g.

 Here , the  Measurement of Uncertainty  ±0.05g. explains that Random error always occur when taking measurements reading with any scale.This tells us that the object's correct reading is between 5.15 g and 5.25 g from 5.20 g- 0.05g. and 5.20 g + 0.05g respectively.

See here on measrement of uncertainty:  link

Atomic number.

Because 2 values given here are 0.05g and one is 5.20g. So there are 2 atoms of one element or compound and one atom of another element or compound.