These problems stumped me in high school but I enjoy working them now.
The first worker can sort the stack in 20 minutes. So every minute that goes by, he does 1/20 of the job.
Likewise, for every minute, the second worker does 1/25 of the stack and the third does 1/100 of the job per minute.
The question is how many minutes will it take if they all work together. Set up an equation that shows what they accomplish each minute.
x stands for minutes and I'm setting it all equal to 1 because this represents that 1 entire stack was sorted.
x·1/20 + x·1/25 + x·1/100 = 1
Multiply each term by 100 to clear the equation of all fractions
100x/20 + 100x/25 + 100x/100 = 100
5x + 4x + 1x = 100
10x = 100
x = 10
The answer is 10 minutes