if the earth be one half of the present distance from the sun, the numvber of days in one year will be a. 129...... is ans

From Kepler's laws (third law actually),

The squares of the sidereal periods of planets = cube of the semi-major axis

For Earth, the semi-major axis = 1 AU and one sidereal period = 1 year.

If the distance is half, ie., 0.5 AU, the period will be

P = sort(0.5^3) = 0.35 year (I am neglecting the extra decimals here).

                             ~ 129 days.

What this mean is, forgetting all the other effects the number of days in the year will be around 129 days.