Step 
<u>find the mass of a single washer</u>
Divide the total mass by the number of washers


Step 
<u>Convert kg to mg</u>
we know that

so

therefore
<u>the answer is</u>

Answer:
Wavelenght is 7,79x10⁻⁵ m
Explanation:
The equation that connects wavelentgh (λ) and frequency (ν) is:
λ=c/ν
Where c is speed of light (3x10⁸ m/sec) and λ is expressed in lenght´s units and ν is expressed in "time⁻¹ " units (for example, sec⁻¹)
According to the details, if we just replace the given value of frequency, we just obtaing wavelenght data:
λ= (3x10⁸ m/sec)/(3,85x10¹² sec⁻¹) = 7,79x10⁻⁵ m
If theres a mixture of components we can calculate the mole fraction
mole fraction can be calculated as follows
mole fraction of component =

number of moles of ethanol - 3.00 mol
total number of moles in mixture - 3.00 + 5.00 = 8.00 mol
mole fraction of ethanol =

mole fraction of ethanol is 0.375