Answer:
7/16 of a yard
Step-by-step explanation:
Let's put the boards to the same denominator so it's easier.
Let's go with a denominator of 16
1/4= 4/16
7/8= 14/16
3/16 stays the same.
So, 4/16+3/16+x=14/16
7/16+x=14/16
x=7/16 of a yard, the length of the 3rd board.
