Challenging P6 2011 Nanyang SA2 P2 Q14

(a) 16, 17, 18, 19, 20, 21

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10
= (1 + 10)/2  x 10 = 55
{Aveage x Number of numbers}
{Average = Sum of first and last numbers divided by 2}

(b) It is 55

1 + 2 + 3 + ..... + 99 + 100
= (1 + 100)/2 x 100 = 5050

5050 - 99  =  4951

c) It is 4951.