let

x = the number of days the girl needs to make 1 dress

1/x dresses per day is the rate of the girl

4 girls in a sewing class can make a dress for 5 days thus their together rate is 1/5 dresses per day

now we have, assuming that each girl has the same rate:

4(1/x) = 1/5

by solving we find:

x = 20 days

click here to see the step by step solution of the equation:

1 girl needs 20 days to make 1 dress.