Final answer:
To find a20, use the formula a20 = a13 + 7(a40 - a13)/20 and plug in the given values to get a20 = -88.75.
Step-by-step explanation:
To find a20, you can use the formula:
a20 = a13 + 7(a40 - a13)/20
Plug in the given values:
a13 = -59, a40 = -144
a20 = -59 + 7(-144 - (-59))/20
a20 = -59 + 7(-144 + 59)/20
a20 = -59 + 7(-85)/20
a20 = -59 - 595/20
a20 = -59 - 29.75
a20 = -88.75