Future rate to calcualte no of contract (MJ21)

In MJ21, Q3, the examiner use rate 0.2374 to calculate the number of contract, but as your lecture , I understand that rate 0.2378 should be used.
Please help me to explain what rate should be used ? Although the result for 2 ways is the same due to rounding, I still would like to know what rate is correct.

Thank you.