英文数字中的[tk]是什么意思?

英文数字中的[tk]是什么意思?正在读一篇有关投资的文章,其中在数字后面多处出现[tk],如“If Yale had stayed half in domestic equities, forty percent in treasuries and ten percent miscellaneous, the endowment would have been at only $10 [tk] billion. Swensen managed to increase Yale’s return by nearly 6% [tk] annually, without increasing its risk.”请问这里的[tk]是什么意思呢?谢谢.