Tire pressure monitoring systems (TPMS) warn the driver when the tire pressure of the vehicle is 27% below the target pressure. Suppose the target pressure of a certain car is 31 psi (pounds per square inch).

a. at what psi will the TPMS trigger a warning for this car?

I got when the tire pressure is below 22.63 psi.

b. suppose tire pressure is a normally distributed random variable with a standard deviation equal to 3 psi. If the car's average tire pressure is on target, what is the probability that the TPMS will trigger a warning?

I got 0.0000 which is incorrect

c. the manufacturer's recommended correct inflation range is 29 to 33 psi. Assume the average tire's psi is on target. If a tire on the car is inspected at random, what is the probability that the tire's inflation is within the recommended rating?

I got .6826 which is incorrect

Please help me understand how to solve b and c. Thanks in advance!