The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach? Choose the correct answer:  A) 31 kg  B) 61 kg  C) 60 kg  D) 62 kg  E) 91 kg
Explanatory Answer   The new average weight of the group after including the football coach = 31 kg.   As the new average is 1 kg more than the old average, old average without including the football coach = 30 kg.   The total weight of the 30 friends without including the football coach = 30 * 30= 
<gadget id="calculator">30 * 30</gadget>
<output>900</output>
900.   After including the football coach, the number people in the group increases to 31 and the average weight of the group increases by 1 kg.   Therefore, the total weight of the group after including the weight of the football coach= 31 * 31= 
<gadget id="calculator">31 * 31</gadget>
<output>961</output>
961 kg.   Therefore, the weight of the football coach= 961 - 900= 
<gadget id="calculator">961 - 900</gadget>
<output>61</output>
61 kg.   Answer 61 kg
<result>B</result>
The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach?