Question

A box contains 20 balls out of which 10 are blue and 10 are green. The balls are drawn at random from the box one at a time with replacement. The probability that a blue ball is drawn 4^th time on the 7^th draw is :

• A. $\frac { 27 } { 32 }$
• B. $\frac { 5 } { 64 }$
• C. $\frac { 5 } { 32 }$
• D. $\frac { 1 } { 2 }$

Answer 1

Answer: (C)

$\frac { 5 } { 32 }$

Answer 2

$\left[ { } ^ { 6 } \mathrm { C } _ { 3 } \times \left( \frac { 10 } { 20 } \right) ^ { 3 } \times \left( \frac { 10 } { 20 } \right) ^ { 3 } \right]$ × $\frac { 10 } { 20 }$ **=**20 ×$\left( \frac { 1 } { 2 } \right) ^ { 7 }$

= $\frac { 5 } { ( 2 ) ^ { 5 } }$ = $\frac { 5 } { 32 }$