Janet can mix a batch of cookie dough in 30 minutes. Miya can mix a batch of cookie dough in 45 minutes. Bella can’t cook, and slows down everybody he works with to half-speed. How many minutes will it take the three of them working together to mix a batch of cookie dough?

Answer 1

Janet's rate ---- 1/30 batches per minute, but with Bella there only 1/60
Miya's rate ---- 1/45 bpm, but with Bella there only 1/90

combined rate = 1/60 + 1/90 + 0 = 1/36
time with all 3 working = 1/(1/36) = 36 minutes

Strange wording and strange question. You mean all 3 have their hands in the same batch of dough?
Just because Bella can't cook would have no effect on his ability to mix dough.

Answer 2

1/T = 1/(2*30) + 1/(2*45).
1/T = 1/60 + 1/90 = 3/180 + 2/180 = 5/180,
T = 180/5 = 36 min.

Or T = 60*90/(60+90) = 36 min.