Relative molecular mass of CuSO\(_4\) = 64 + 32 + (16 x 4) = 160
The number of moles of CuSO\(_4\) = \(\frac{mass}{Molar mass}\)

  = \(\frac{20}{160}\)

 n = 0.125 mole
100g of water dissolved 0.125mole of CuSO\(_4\)
100g of water will dissolve \(\frac{0.125}{100}\times{1000}\)

  Solubility = 1.25 moldm\(^{-3}\)

Alternatively, 

Solubility = Number of moles of solute x  \(\frac{1000}{Volume of solvent}\)

But Number of moles of solute = \(\frac{mass}{molar mass}\) = \(\frac{20}{160}\) = 0.125 moles

Solubility = Number of moles of solute x  \(\frac{1000}{Volume of solvent}\)

Solubility = 0.125 x  \(\frac{1000}{100}\)

 = 1.25 moldm\(^{-3}\)