Answer
19 or more additional Senators must vote in favor in order to ratify.
Work Step by Step
$\frac{2}{3}\times100=66.\overline6$
67 Senators must ratify a treaty.
Assume that "a" is the additional Senators who need to vote in favor of the treaty.
$a+48\geq67$
$a+48-48\geq67-48$
$a\geq19$
