please explain this question in detail especially b part.

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Question 6 of 6 Gavin and Holly purchased a $740,000 condominium in Toronto. They paid 20% of the amount as a down payment and secured a 25-year mortage for the balance. They negotiated a fixed interest rate of 3.7% compounded semi-annually for a 5-year term with repayments made at the end of every month. Their mortgage contract also stated that they may prepay up to 15% of the original principal every year without at interest penalty. At the end of the first year, in addition to the regular monthly payment, they made a lump-sum payment of $24,000. a. What was the size of the monthly payment? $3,018.52 Round to the nearest cent b. What was the principal balance at the end of the first year, prior to make the lump sum payment? $553,268.57 Round to the nearest cent c. By how much did the amortization period shorten after they made the lump sum payment at the end of the first year? 18 months