Martin:
17 7/8 ÷ 3 2/5 → 16 ÷ 4 = 4
Tina:
17 7/8 ÷ 3 2/5 → 18 ÷ 3 = 6
Tina made the better estimate. Explanation :
7/8 is closer to 1 than it is to 0. ⇒ This means that the better estimate is to see 17 7/8 as 17 + 1 and round it as 18.
2/5 is closer to 0 than it is to 1. ⇒ This means that the better estimate is to see 3 2/5 as 3 + 0 and round it as 3.
There you go! I really hope this helped, if there's anything just let me know! :)