Every day Jo practices her tennis serve by continually serving until she has had a total of 50 successful serves.?

P(she will need more than 100 serves to have a total of 50 successful serves)

= P(she have a total of 0 successful serves in the first 100 serves)
+ P(she have a total of 1 successful serves in the first 100 serves)
+ P(she have a total of 2 successful serves in the first 100 serves)
+ ......
+ P(she have a total of 49 successful serves in the first 100 serves)

= 100C0 * 0.4º * 0.6¹ºº + 100C1 * 0.4 * 0.6^99 + 100C2 * 0.4² * 0.6^98 + ... + 100C49 * 0.4^49 * 0.6^51
= Σ (n = 0 to 49) 100Cn * 0.4ⁿ * 0.6¹ºº⁻ⁿ
= 0.9729...