A particle moving with constant acceleration covers a distance of 20m in 3rd sec and 40m in 5th sec. What is acceleration of the particle?

Distance covered in nth  sec Sn=U+an−12  here initial velocity=U;acceleration=aIn⁡3rd sec⁡ Distance covered S3=U+a3−12⇒20=U+5a2………..(1) In 5th sec⁡ Distance covered S5=U+a5−12⇒40=U+9a2……....(2)subtract equation(2)−(1)⇒40−20=9a2−5a2⇒20=a2(9−5)⇒20=a2(4)⇒a=10m/s2