Question $2$

You are buying a condo:

The condo is $3$ billions VND

Bank $1$ gives you the interest of $11\%$ for $15$ years, with $30\%$ down payment.

$2\mathrm{.}1$billion

Bank $2$ gives you the interest rate of $8\%$ for $20$ years, with $10\%$ down payment

$2\mathrm{.}7$ billion

Bank $3$ gives you the interest rate of $6\%$ for $30$ years, with $0\%$ down payment.