Final answer:
The length of B'C' after a dilation with a scale factor of 1/4 is 7.5 cm.
Step-by-step explanation:
If a dilation maps triangle ABC onto triangle A'B'C' and we know that AC = 24 cm and A'C' = 6 cm, then the scale factor of the dilation is 6/24, which simplifies to 1/4. To find B'C', we use the same scale factor, because dilations maintain proportionality. Since BC = 30 cm, then B'C' can be calculated as follows:
B'C' = BC × Scale Factor = 30 cm × (1/4) = 7.5 cm.
Therefore, after the dilation, the length of B'C' is 7.5 cm.