Measure showing different sum result with multiple selection

[RPAI2812]

I have a measure that associates the daily rate of a contract and the actual use of that contract in a month.

Measure 1: Day rate (USD/day) = SUMX('Table 1','Table 1'[Day Rate])
Measure 2: Used days (day) = SUMX('Table 1','Table 1'[Used days])
Measure 3: Rate cost = [Day rate (USD/day)]*[Used days (day)]

I have different contracts and when I select them individually I have the correct rate cost. However, when I multiple select contracts in the slicer, the results is NOT summing correctly the rate cost.

For instance:
Contract 1 rate cost = 100K USD
Contract 2 rate cost = 200K USD
Contract 1 + Contract 2 rate cost should be 300K USD, but it shows a different result.

Where am I making a mistake?

[SteveHailey · Accepted solution]

Hey @RPAI2812.

Try: 

Rate Cost =
SUMX (
    'Table 1',
    'Table 1'[Day Rate] * 'Table 1'[Used Days]
)

That will go row by row and multiply the rate times the days, and then sum those values, which is correct.

Your measures are instead summing the rates, summing the days, and then multiplying those two sums together, which is incorrect.