A$50,000 investment earns 18% annually. if b represents the balance and t represents the number of years, determine an expression that gives the account balance, b, after t years. also, determine an equivalent expression that displays the effective monthly interest rate.

Using the compound interest formula;
FV=P(1+r/100)^t

FV is the future value
P is the principle 
r is the rate
t is the time

the formula that gives the account balance after t years will be:

FV=50000(1+18/100)^t
FV=50000(1.18)^t

The formula for effective monthly interest rate will be:
monthly interest rate=(annual rate)/(number of months)
annual rate=18%
number of months in a year=12 months
rate=18/12=1.5%