An oxide of metal have 20% oxygen, the equivalent weight of metal oxide is

20% Oxygen implies 20 g Oxygen combined with 80 g of metal.

Chemical substances combine in terms of equal equivalents.

No of equivalents of Oxygen=$\frac{Mass of Oxygen 20g}{Mass of one equivalent of Oxygen 8g}=2.5 equivalents$

2.5 equivalents of Oxygen will combine with 2.5 equivalents of the metal that weigh 80g.

Equivalent weight of metal=$\frac{Mass of Metal 80g}{No. of g equivalents 2.5}=32$

Mass of one equivalent of Metal Oxide= one equivalent of metal 32 + one equivalent of Oxygen 8 = 40