8) C

Upper quartile means 75% to 100%.

Difference = 100 – 75 = 25%

Probability of a person having a heart beat of 165 or more = 25% ÷ 100% = ¼

4 members were chosen. 3 would have probability of ¼ and the fourth would have probability of ¾.

Possibilities of the order of people = YYYN, YYNY, YNYY, NYYY

Y = person has a heart beat of more than 165

N = person has a heart beat of less than 165

So there are 4 possibilities.

Probability of 3 members having heart beat 165 or more = ¼ x ¼ x ¼ x ¾ x 4 = ¹²⁄₂₅₆

The question says __at least__ 3 people. So we can also obtain 4 people with a heart beat of more than 165.

Probability of this = ¼ x ¼ x ¼ x ¼ = ¹⁄₂₅₆

Probability of at least 3 people having a hear beat of more than 165 = ¹²⁄₂₅₆ + ¹⁄₂₅₆ = ¹³⁄₂₅₆