Table:
|  | Cost | Accum. amort. | Net book value |
| --- | --- | --- | --- |
| Assets subject to amortization: |  |  |  |
| Customer related intangibles | $3,926.8 | $(1,083.6) | $  2,843.2 |
| Unpatented technology | 504.0 | (199.5) | 304.5 |
| Software | 172.0 | (93.2) | 78.8 |
| Patents and other protective rights | 9.7 | (7.5) | 2.2 |
| Trade names | 7.3 | (2.8) | 4.5 |
| Assets not subject to amortization: |  |  |  |
| Trade names | 608.9 | — | 608.9 |
| Balances at December 31, 2018 | $  5,228.7 | $(1,386.6) | $  3,842.1 |
| Assets subject to amortization: |  |  |  |
| Customer related intangibles | $  4,955.4 | $(1,349.4) | $  3,606.0 |
| Unpatented technology | 613.0 | (279.6) | 333.4 |
| Software | 172.2 | (111.5) | 60.7 |
| Patents and other protective rights | 12.0 | (8.0) | 4.0 |
| Trade names | 7.9 | (4.1) | 3.8 |
| Assets not subject to amortization: |  |  |  |
| Trade names | 659.8 | — | 659.8 |
| Balances at December 31, 2019 | $ 6,420.3 | $(1,752.6) | $ 4,667.7 |
Paragraphs:
Other intangible assets were comprised of:
Amortization expense of other intangible assets was $364.7, $316.5, and $294.3 during the years ended December 31, 2019, 2018 and 2017, respectively. Amortization expense is expected to be $400 in 2020, $383 in 2021, $379 in 2022, $347 in 2023 and $321 in 2024.
Question:
What is the percentage change in balances of net book value from 2018 to 2019?
Answer: (4667.7-3842.1)/3842.1  = 21.49