Part A is 2.5 x

because the decimal place has been moved five times to the left, which makes it negative.
Part B is
First, find what the diameters of both things are written out.
1 x

= .00001
1 x

= .0001
Now find out how many times the size of a cell has to be multiplied to equal the diameter of a grain of sand.
.00001 x 10 = .0001
.0001 = 1 x

So the diameter of a grain of sand is 10 times longer than the diameter of the average cell in the human body.