David has some sushi. Andy has 8 times as much sushi as David. Terissa has 4 times as much sushi as David. Together, they have 78 pieces of sushi
amount of sushi that David has =====> x
"Andy has 8 times as much sushi as David" ====> .......
"Terissa has 4 times as much sushi as David" ===> ....
"Together, they have 78 pieces of sushi"
====> (their sum) = 78
solve for x