$\text { Đáp án: }$
` a) `
` x : 15 = 8 : 24 `
` => x × 24 = 8 × 15 `
` => x × 24 = 120 `
` => x = 120 : 24 `
` => x = 5 `
$\text { Vậy }$ ` x = 5 `
` b) `
` 3 1/2 : 0,4 = x : 1 1/7 `
` => 7/2 : 2/5 = x : 8/7 `
` => x × 7/8 = 7/2 × 5/2 `
` => x × 7/8 = 35/4 `
` => x = 35/4 × 8/7 `
` => x = 10 `
$\text { Vậy }$ ` x = 10 `