Force = mass x acceleration

but force was halved = 1/2 and mass doubled = 2 ( assuming that force and mass are 1N and 1kg respectively)

f = ma = \(\frac{1}{2}\) = 2 x a 

dividing both sides by 2 

a = \(\frac{1}{2} \times\frac{1}{2} = \frac{1}{4}\)