Answer:
Step-by-step explanation:
After 20 minutes, will all of them working together, they would have typed 13 letters. Then after another 20 minutes, it would be 26. Then 39. But after that is gets tricky. Mary types a letter every 3 minutes. Maggie types a letter every four minutes. and Su Ying types a letter every 10 minutes. After 6 minutes, the total would go to 42 letters. Then another 3 minutes it would be 44 letters. And finally, Su Ying finishes it. Taking a total of 70 minutes, or 1 hour and 10 minutes.
Mark Brainliest Please, And I'm Glad I Could Help. (If I Did) :) Have A Good Day!