Although you are unlikley to see such a calculation on the exam, if you want to calculate MWRR on BA II Plus there is a cheap way.
Invest 900,000 at day 0
Invest 50, 000 at day 15
Get 1,466,553 on day 3
Use the cash flow worksheet, set CF0 to 900,000
Set CF1 to 0, and its frequency to 14
Set CF2 to 50,000 and its frequency to 1
Set CF3 to 0 and its frequency to 14
Set CF4 to -1,466,553 and its frequency to 1
The resulting MWRR is your daily MWRR, you would wont it for the total period so raise it to the power of 30 in this case. (1+r)^30
When the dates are split out equally like in this case, it would have been easier to just use CF1 and CF2, the result would be a 15 day MWRR, but if you get cashflows that are not even, ie day 7, 12, 19, then the method above will be your choice.