Answer:
Less than 21 cm.
Explanation:
Let x represent the multiplier.
We have been given that the lengths of the sides of a triangle are in the ratio 5:6:7. The perimeter of all sides of the triangle would be
.
We have been given that the perimeter is less than 54 cm.
We can set this information in an inequality as:




Since the length of longest side is
, so:



Therefore, the length of the longest side will be less than 21 cm.