A boy is twice as tall as his little sister and 30cm shorter than his father. Their combined height is 3.8 meters. Find (in cm) the height of each person.

Call the sister's height x. The boy is twice as tall, so that's 2x. The father is 30 cm taller than the boy, so his height is 2x + 30. The combined height is 3.8m, which is 380cm. Now you can write an equation

x + 2x + 2x + 30 = 380

collect like terms

5x = 350

x = 70

the sister is 70cm tall, the boy is 2*70 = 140 cm, and the father is 140 + 30 = 170cm

Check that the heights add up to 3.8m: 70 + 140 + 170 = 380 cm = 3.8m. Correct

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