Define A = The sum of digits of the number 7^100 and B = The sum of the digits of A. What is the sum of the digits of B?
Choose the most appropriate option.
A) 7 B) 16 C) 10 D) 11 E) Cannot be Determined
Following approach will take you only couple of sec.   \(7^{100} = (7^{50})^2\)   The sum of digits (digital root) of a PERFECT SQUARE can only be 1, 4, 7 or 9.   For example \(7^2=49\) =&gt; DR=4+9= 
<gadget id="calculator">4+9</gadget>
<output>13</output>
13=&gt; 1+3= 
<gadget id="calculator">1+3</gadget>
<output>4</output>
4 and etc.   Checking available options.   7nswer 7.
<result>A</result>
Define A = The sum of digits of the number 7^100 and B = The sum of the digits of A. What is the sum of the digits of B?