Went back and tried these again, still no luck. A sound wave travels at a frequency 2.25 MHZ(10^6) through a pregnant woman's abdomen and is reflected from the fetal heart wall of her unborn baby. The heart wall is moving toward the sound receiver as the heart beats. The reflected sound is then mixed with the transmitted sound, and 87.0 beats per second are detected. The speed of sound in body tissue is 1510 m/s. Calculate the speed of the fetal heart wall at the instant this measurement is made. I come up with: [1510(((2.25*10^6)+87)/(2.25*10^6))]-1510 m/s Does anyone see an error in this? Next, A certain pipe produces a fundamental frequency of f in air. If the pipe is filled with helium at the same temperature, what fundamental frequency does it produce? (Take the molar mass of air to be M_air, and the molar mass of helium to be M_He.) I get: (f*sqrt(M_air))/sqrt(M_He))) This is off by a multiplicative factor... Does anyone see my error?