physics problems

1.According to the Newton 2nd law of motion,

F=ma
F=m (v-u)/t

applying this equation, as you mention the initial velocity and the final velocity remain the same with only the time required is shortened, the accrelation rate should be increased.
It is a mathematics knowledge, with denominator decrease, the whole figure will increase. e.g. x/y, a smaller y will result a larger value of the whole equation x/y.

2. Your question cannot be simply solved with not enought information provided. It should be depended on the direction of the force.
If two force is in the same direction, it would be the case for a simple addition,
e.g. 5N + 10N = 15N
If two force is in opposite direction, it would be a case of subtraction,
e.g. 50N - 20N = 30N
However, if the two force is neither on the same direction nor opposite direction, it would be a problem of restoring force which involve the use of sin, cos or tan. The resultant force would be depended on the angle made between two forces.
e.g. if they are perpendicular to each other, the recultant force would be:
(20N)cos45+(20N)cos45 = (20N)( 2/square root 2)
A angle between 2 forces would affect the choice of answer.

i hope the answer would be helpful for you