What is the simple interest formula to use if a family borrowed 900 dollars at 15 percent annual interest for 1 year and 8 months so how much did they have to repay at the end of time?

The simple interest formula is ( I = P \times r \times t ), where ( I ) is the interest, ( P ) is the principal amount (900 dollars), ( r ) is the annual interest rate (15% or 0.15), and ( t ) is the time in years (1 year and 8 months equals 1.67 years). Calculating the interest: ( I = 900 \times 0.15 \times 1.67 \approx 225.15 ). Therefore, the total amount to repay is ( 900 + 225.15 \approx 1125.15 ) dollars.