What will be the total damage calculated if you used Muddy Water + Rain Dance?
While Rain Dance is in effect, the base damage of Water-type moves will be increased by 50%, whereas the base damage of SolarBeam and Fire-type moves will be decreased by 50%. -Bulbapedia
Muddy Water's base power is 90. Half of 90 is 45. 90 + 45 = 135.