Instruction 12 -Referring to Instruction 12

Instruction 12.39
The managers of a brokerage firm are interested in finding out if the number of new customers a broker brings into the firm affects the sales generated by the broker. They sample 12 brokers and determine the number of new customers they have enrolled in the last year and their sales amounts in thousands of dollars. These data are presented in the table that follows.
$$\begin{array} { | l | l | l | } \hline \text { Broker } & \text { Clients } & \text { Sles } \\\hline 1 & 27 & 52 \\\hline 2 & 11 & 37 \\\hline 3 & 42 & 64 \\\hline 4 & 33 & 55 \\\hline 5 & 15 & 29 \\\hline 6 & 15 & 34 \\\hline 7 & 25 & 58 \\\hline 8 & 36 & 59 \\\hline 9 & 28 & 44 \\\hline 10 & 30 & 48 \\\hline 11 & 17 & 31 \\\hline 12 & 22 & 38 \\\hline\end{array}$$
-Referring to Instruction 12.39,suppose the managers of the brokerage firm want to obtain a 99% prediction interval for the sales made by a broker who has brought into the firm 18 new customers.The t critical value they would use is __________.