A particle is moving Eastwards with a velocity of $5m{s}^{-1}.$ In 10 s, the velocity changes to $5m{s}^{-1}$ Northwards. The average acceleration in this time is 

As, velocity of Eastward $v_1=+5\widehat{i}$  
       
And velocity of Northward $v_2=+5\widehat{j}$  
$\Delta v=v_2-v_1=5\widehat{j}-5\widehat{i}$ 
$\left|\Delta v\right|=\sqrt{5^2+5^2}=5\sqrt{2}$ 
$\therefore a=\frac{\left|\Delta v\right|}{t}=\frac{5\sqrt{2}}{10}=\frac{1}{\sqrt{2}}m{s}^{-2}$ 
For direction, $\tan \alpha =-\frac{5}{5}=-1$  
Average acceleration is $\frac{1}{\sqrt{2}}m{s}^{-2}$ towards North-West.