1/5x = 3/7 y
(let pineapple pies be x and apple pies be y)
solve simultaneous equations
let pineapple as X, apple as Y.
1/5 X = 3/7 Y (1)
Solve the equation to get X=900, and Y=430
Think there’s a typo. Based on your equations, it should be Y = 420.