What do you add to 3 7/8 to make 7

You add 3 1/8 to make 7 because 3 + 3 is 6 and then 1/8 + 7/8 is 1; therefore, 6 + 1 is 7.

The easiest way would be to subtract (3 and 7/8) from (7) to get (3 and 1/8). Another way would be to convert (3 and 7/8 to 31/8) and (7 to 56/8) and then subtract straight acrossed the top to get (25/8 which when converted to a mixed number is 3 and 1/8). If i confused you or i need to better explain don't be afraid to ask.