I=PRT means that P=I/(RT)=48/(0.04*0.75)=48/.03=4800/3=1600. So P=$1600. If you don't like decimal fractions, let's look at R and T in a different way. The annual (12-monthly) rate is 4%. So for 9 months (9/12=3/4) we reduce the percentage to 3/4 of 4%=3% or 3/100. So RT=3/100. To divide by this fraction we invert it and multiply by 100/3, so to multiply the interest by 100/3, 3 goes into 48 16 times multiplied by 100 is 1600, making the principal amount $1600.