Remark
This is an arithmetic progression. You have the first and last terms and the difference.
Givens
a = - 18
L = 60
d = 4.5
Equation
L = a + (n - 1)d
Solve
60 = -18 + (n - 1) * 4.5 Add 18 to both side
60 + 18 = (n - 1) * 4.5
78 = (n - 1) * 4.5 Divide both sides by 4.5
78/4.5 = n - 1
17.3333 = n - 1 Add 1 to both sides.
18.3333 = n
Conclusion
It will that 18 1/3 or 18.33333 minutes to get the from -18 to 60 degrees.
