The answer is:
Rounded to the nearest whole number, there are 40 electrons in an atom of Zirconium.
I hope this helps :D
In this question, the thing that needs to be found is the number whose 20% would be 18. The information's required for getting to the solution are already provided in the question. based on those given information's, the answer can be deduced very easily.
Let us assume the unknown number to be = x
Then
(20/100) * x = 18
x/5 = 18
x = 18 * 5
= 90
so the unknown number in the question is 90. I hope the procedure is clear enough for you to understand.
27; The first stack is 3 x 3 (= 9). You then multiply 9 by 3 (for each of the 3 stacks). Basically, 3 x 3 x 3.
