Description for Correct answer:
Let the lengths of parallel sides an 3x and 2x.

We know that,

Area of trapezium = \( \Large \frac{1}{2} \) \( \Large (Sum \ of \ the \ parallel \ sides)\times Distance \ between \ them \)

\( \Large \frac{1}{2}(3x+2x)15=450 \)=>75x=900

x=\( \Large \frac{900}{75} \)=12 cm

Sum of the parallel sides = (3x + 2x)

= 5x = \( \Large 5\times 12 \)=60 cm