Final answer:
The balance in the pension plan asset fund as of December 31, 2020, is $429,000.
Step-by-step explanation:
To calculate the balance in the pension plan asset fund, we need to consider the service cost, the amount funded, and the actual rate of return. The balance in the pension plan asset fund is equal to the amount funded plus the return on investments. In this case, the amount funded in 2019 is $240,000, and in 2020 is $390,000. The actual rate of return is equal to the expected rate of return, which is 10%. So, the balance in the pension plan asset fund as of December 31, 2020, is $390,000 + ($390,000 x 10%) = $429,000.