book Maths Puzzle Numerical Ability

The half-life of Bismuth is 5 days. Then 75% of an original amount will cease to be radioactive in 10 days. I believe that this one is true. Am I right?

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book Other Question

1) 3x-2y=4
2x-5y=21

2) 4x+5y=-5
3x+y=21

3) 3x+y=7
3x+y=10

I cannot find the solutions to these systems. PLEASE HELP! I have to have them due soon! Curly can do a job in 30 min, Larry can do it in 40 min, and Moe can do it in 60 min. How long will it take them if they work together?