Ethanol, as needed to obtain methyl ethyl 30 g of ether.

Given:
m (CH3-O-C2H5) = 30 g

To find:
m (C2H5OH) -?

Decision:
1) CH3OH + C2H5OH = (120 ° C, H2SO4) => CH3-O-C2H5 + H2O;
2) M (C2H5OH) = Mr (C2H5OH) = Ar (C) * N (C) + Ar (H) * N (H) + Ar (O) * N (O) = 12 * 2 + 1 * 6 + 16 * 1 = 46 g / mol;
M (CH3-O-C2H5) = Mr (CH3-O-C2H5) = Ar (C) * N (C) + Ar (H) * N (H) + Ar (O) * N (O) = 12 * 3 + 1 * 8 + 16 * 1 = 60 g / mol;
3) n (CH3-O-C2H5) = m (CH3-O-C2H5) / M (CH3-O-C2H5) = 30/60 = 0.5 mol;
4) n (C2H5OH) = n (CH3-O-C2H5) = 0.5 mol;
5) m (C2H5OH) = n (C2H5OH) * M (C2H5OH) = 0.5 * 46 = 23 g.

Answer: The mass of C2H5OH is 23 g.