`100/x + 200/x + 300/x + 400/x + 500/x = 15`
`100 : x + 200 : x + 300 : x + 400 : x + 500 : x = 15`
`100 × 1/x + 200 × 1/x + 300 × 1/x + 400 × 1/x + 500 × 1/x = 15`
`(100 + 200 + 300 + 400 + 500) × 1/x = 15`
`1500 × 1/x = 15`
`1/x = 15 : 1500`
`1/x = 1/100`
`x = 100`
Vậy `x = 100`
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Cách khác:
`100/x + 200/x + 300/x + 400/x + 500/x = 15`
`(100 + 200 + 300 + 400 + 500)/x = 15`
`1500/x = 15/1`
`x × 15 = 1500 × 1`
`x × 15 = 1500`
`x = 1500 : 15`
`x = 100`
Vậy `x = 100`