Is the textbook answer for acceleration and distance correct?

[Afro_Akuma]

Here's the problem. A force F gives an acceleration to an object of a given mass.

a) If this mass is doubled and five times the force is applied, what is the current acceleration of the object.

b) How will this change the distance covered by this object over a given interval of time?

My textbook gives the answer for part b) as d'=2.5d

However, When I set a time and an initial velocity and work it out, the result doesn't come to 2.5. Am I screwing up or is my textbook wrong?

 

[Leong]

F=ma
a=\frac{F}{m}
5F=2ma'
a'=\frac{5F}{2m}
a'=\frac{5}{2}*a
Assuming the initial velocity of the mass, u = 0.
d=\frac{1}{2}at^2
d\ '=\frac{1}{2}a't^2
d\ '=\frac{1}{2}*\frac{5}{2}*a*t^2
d\ '=\frac{5}{2}*d

 

[Afro_Akuma]

Yes, but the problem doesn't state that the object is at rest, so can we just automatically assume that it is?