When you say this you can do one of two things:
1) You can turn 2/5 into a decimal, which would make the equation 0.4+3.5=3.9
2) You can turn 3.5 into a fraction, which would be 3 1/2. Find a common denominator for 3 1/2 and 2/5. The least common denominator is 10. 3 1/2 turns to 3 5/10. 2/5 turns to 4/10. 3 1/2+4/10+3 9/10.
Either way, you get the same answer, and my work here is done
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