Inverse/Direct Proportionality help please!

Hello all

Geez i dunno if this is even direct or inverse proportionality, or if this threads even in the right section :S sorry, first time here!

But anyway, to the question..

"A particle moves along a line.

For 't' bigger than or equal to 1, the distance of the particle from 'O' at time 't' seconds is 'x' metres, where:

x=20/t

Find an expression for the acceleration of the particle"

Question taken from an IGCSE paper im stuck on.. dont really understand how to solve it at all

Thanks in advance!