Answer:
Step-by-step explanation:
Remark
I suppose the question is what is Thaddeus' time to make the trip. This is a fairly common question but not everyone knows how to solve it.
Derivation of the Formula
Prescott + Thaddeus = 1 trip
So the logic of the equation is this.
distance = 1 trip
time = number of dates
rate = ?
rate = distance / time
To find out what the contribution of Thaddeus is set up a distance time formula.
1/21 days is the rate for Prescott
1/x days is the rate for Thaddeus
Together the rate is 1/9
Solution
1/21 + 1/x = 1/9
Subtract 1/21 from both sides
1/x = 1/9 - 1/21
It's not obvious what the LCM is.
9 = 3 * 3
21 = 3 * 7
LCM: 7 * 3 *3 = 63
1/x = 7/63 - 3/63 = 4/63
1/x = 4/63 Cross Multiply
4x = 63 Divide by 4
x = 63/4
Answer
x = 15 3/4
x = 15.75
