Its 81 centimeters because you round 5 and over. So nearest centimeter means 2 whole numbers which is 81
The easiest way is if it doesn't need to be simplified. Multiply -2.105 by

. You get the fraction

. While this is technically the answer, you can simplify this down to

. Because 421 is a prime number, this can not be simplified any further. Therefore, the solution is

.

.
So he started with 3/4 lb and ended up with 2/3 of that so 2/3 of 3/4
'of' can be roughly translated as multiply
so 2/3 times 3/4 =6/12=1/2 lb left
or you know that there are 3 out of 4 parts pound left so 2 out of 3 parts of nuts is = 2/4 or 1/2 lb