The mineral enargite is 48.41% cu, 19.02% as, and 32.57% s by mass. what is the empirical formula of enargite?

Empirical formula is the simplest ratio of whole numbers of components in a compound. 
Assuming for 100 g of the compound 
                                Cu                                 As                             S
mass                      48.41 g                          19.02 g                      32.57 g
number of moles    48.41 / 63.5 g/mol      19.02 / 75 g/mol        32.57 / 32 g/mol 
                                = 0.762 mol                = 0.2536 mol            = 1.018 mol 
divide by the least number of moles 
                               0.762 / 0.2536             0.2536 / 0.2536         1.018 / 0.2536
                               = 3.00                          = 1.00                         = 4.01
once they are rounded off 
Cu - 3
As - 1
S - 4
therefore empirical formula is Cu₃AsS₄

The empirical  formula  of enargite  is calculated as below

find the moles of each element
that is moles =% composition/molar mass

=Cu = 48.41/63.5 =0.762  moles
As = 19.02/74.9 = 0.254 moles
S =32.57/32.1 = 1.015 moles

find the moles ratio by diving  each  mole with the smallest mole( 0.254 moles)

CU =0.762/0.254 = 3
 As =0.254/0.254 =1
S= 1.015/0.254 = 4

therefore the empirical formula  =Cu3AsS4

668900 atoms in total