If you would like to know how much gas did Ivy's father purchase during the third week, you can calculate this using the following steps:
22.5 gallons + 19.71 gallons + n = 61.75 gallons
n = 61.75 - 22.5 - 19.71
n = 19.54 gallons
Result: Ivy's father purchased 19.54 gallons during the third week.