find moles of MgSO4.7H2O
molar mass = 246
so moles = 32 / 246 = 0.13 moles.
When heated, all 7 H2O from 1 molecule will be gone.
total moles of H2O present = 7 x 0.13 = 0.91
mass of those H2O = 0.91 x 18 = 16.38g
so mass of anyhydrous MgSO4 remain = 32 - 16.38 = 15.62 g