Since ∠ACB and ∠BCD are supplementary angles, their measures add up to 180°. Therefore, we can set up the following equation:
\[ (x + 28) + (x + 48) = 180 \]
Combining like terms gives:
\[ 2x + 76 = 180 \]
Subtracting 76 from both sides:
\[ 2x = 104 \]
Dividing by 2:
\[ x = 52 \]
Now, we can find the measure of ∠BCD:
\[ m∠BCD = x + 48 = 52 + 48 = 100° \]
Thus, the measure of ∠BCD is \( \boxed{100°} \).