Amicroscope has a 20 x ocular (eyepiece) and two objectives of 10 x and 3x respectively:
a.) calculate the low power agnification of this icroscope.

Magnification at Low power = 20 x 10 = 200x. 

So, the low power magnification of this microscope is 200times.

The magnification is how many times, the object has been enlarged. To compute the magnification, you will require to increase the magnification of the visual focal point with the magnification of the goal focal point. See underneath for the arrangement.

Magnification=magnification of ocular lens X magnification of objective lens.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it