Answer:
11.9%
Step-by-step explanation:
Total ways to pick 4 computers = 10×9×8×7
Total ways that at least three computers are fine =
Total ways that all 4 are fine + Total ways any 3 are fine
Total ways that all 4 are fine = 4×3×2×1
Total ways three are fine = 1st is Not working and other 3 working +
2nd is Not working and other 3 working +
3rd is Not working and other 3 working +
4th is Not working and other 3 working +
= 6×4×3×2 + 4×6×3×2 + 4×3×6×2 + 4×3×2×6
= 6×4×3×2×4
The probability = Total ways that at least three computers are fine /
Total ways to pick 4 computers
= (4×3×2×1 + 6×4×3×2×4) / (10×9×8×7)
= (4×3×2×25) / (10×9×8×7)
= 11.9%
