In the figure,abcd is a square. angle bce is 4/5 angle dce. find angle dce

Well it is a square so angle BCD is 90 degrees.

Now <BCE + <DCE = <BCD

Let <DCE = β, this means that <BCE=4/5 * β

so

$$\\
\beta + \frac{4}{5}\beta = 90\\\\
\beta \times (1\frac{4}{5})=90\\\\
\beta = 90\div 1\frac{4}{5}\\\\
\beta = 50^0\\\\