The missing fact in the fact family is 5-2=3. In a fact family, the two addition facts and two subtraction facts are related to each other. In this case, the addition facts are 1+3=4 and 3+1=4, while the subtraction facts are 4-1=3 and 4-3=1. The missing fact completes the fact family by showing another subtraction fact where the difference is 3.