Concept:Use coordinate geometry to track the child's movements and find the straight-line distance from start.Explanation:Let the starting point be (0,0).East is +x, North is +y.Step 1: 90 m East → (90,0).Step 2: Turn right (now South), go 20 m → (90,−20).Step 3: Turn right (now West), go 30 m → (60,−20).Step 4: Go 100 m North → (60,80).Distance from start = (60−0)2+(80−0)2​=3600+6400​=10000​=100 m.Answer:100 metres