From the above figure,

Initial velocity, \(\vec{v}_{ i }=\left(-20 \sin 30^o \hat i-20 \cos 30^{\circ} \hat j \right) m / s\)

Final velocity, \(\overrightarrow{ v }_{ f }=\left(-20 \sin 30^{o}\hat i+20 \cos 30^o \hat  j \right) m / s\)

Change in velocity, \(\Delta \vec{v}=\vec{v}_f-\vec{v}_i\)

\(\Delta \overrightarrow{ v }=\left(-20 \sin 30^{o}{ \hat{ i }}+20 \cos 30^0 j \right)-\left(-20 \sin 30^o \hat{ i }-20 \cos 30^{o}{\hat j }\right)\)

\(\Delta \overrightarrow{ v }=2 \times 20 \cos 30^{o}{ \hat{ j }}\)

\(\Delta \overrightarrow{ v }=2 \times 20 \times \frac{\sqrt{3}}{2} \hat{ j }\)

\(\Delta \overrightarrow{ v }=20 \sqrt{3} \hat{ j }\)