Answer:
70 minutes
Step-by-step explanation:
Mary can type the number of letters in 20 minutes = 6
Maggie can type the number of letters in 20 minutes = 5
Su Ying can type the number of letters in 20 minutes = 2
Working together they can type the number of letters in 20 minutes
= 6 + 5 + 2
= 13 letters
Working together they can type the number of letters in 1 minute =
To type 45 letters time taken by them =
=
=
= 69.230 ≈ 70 minutes
It will take them 70 minutes to type 45 letters.