What is the received signal level with a transmit power of 10 W (+40 dBm), a transmit antenna gain of 6 dBi, a receive antenna gain of 3 dBi, and a path loss of 100 dB?
Why This is the basic link budget equation: received level in dBm equals transmit power in dBm plus all gains minus all losses. Here that is +40 dBm + 6 dBi + 3 dBi - 100 dB = -51 dBm. Because everything is already in decibel form, you just add and subtract; no multiplying is needed. Note that 10 W is +40 dBm, since 1 W is +30 dBm and each factor of 10 adds 10 dB.
Watch out The values near -54 dBm and -57 dBm come from leaving out one of the two antenna gains, and -60 dBm comes from ignoring both; every gain in the path counts, at each end.
Add gains, subtract losses, all in dB: 40 + 6 + 3 - 100 = -51. And remember 10 W = +40 dBm.