You paint 1/3 of a wall in 1/4 hour. At that rate, how long will it take you to
paint one wall?

You would have to multiply

Step-by-step  1/4× 4/4 that will equal to 4/16 and then you reduce it so your answer is 1/4                                                                                                                      

Actually it would take 3/4 of an hour.
Explanation: If you did 1/3 of the wall in 1/4 of an hour, then to find how long it would take to finish you would multiply both by 3 because 1/3 x 3 is 1. So 1/4 x 3 = 3/4 which is also 45 minutes.

What do you mean i don’t understand

21 is the answer the answer is 21