Answer
$14\leq k\leq26$
Work Step by Step
$15\leq\frac{20+11+k}{3}\leq19\longrightarrow$ multiply all 3 parts by 3
$15\times3\leq\frac{20+11+k}{3}\times3\leq19\times3\longrightarrow$ multiply
$45\leq31+k\leq57\longrightarrow$ subtract 31 from all 3 parts
$45-31\leq31+k-31\leq57-31\longrightarrow$ subtract
$14\leq k\leq26$
