What is the molecular formula of a compound whose molar mass is 88 and whose percent composition is 54.5% carbon, 9.1% hydrogen, and 36.4% oxygen?

Molecular formula is the actual composition of elements in the compound.
we can first find the empirical formula. empirical formula is the simplest ratio of whole number of elements in the compound .
for 100 g of compound 

                               C                            H                          O
mass                     54.5 g                     9.1 g                  36.4 g
number of moles   54.5 g/12 g/mol     9.1 g/1 g/mol       36.4 g / 16 g/mol 
                              = 4.54 mol              = 9.1 mol            = 2.28 mol
     divide by the least number of moles 
                               4.54 / 2.28  =1.99    9.1/2.28=3.99     2.28/2.28 = 1
numbers rounded off
C - 2
H - 4
O - 1
empirical formula - C₂H₄O
mass of empirical formula - (12*2) + (4*1) + (1*16) = 44 
mass of molecular formula - 88 g/mol

number of empirical units - 88 g/mol / 44 = 2
there are 2 empirical units 
molecular formula - 2 (C₂H₄O)
molecular formua - C₄H₈O₂