39 ÷ 5 = 7.8
Hack:
Multiply the 2 digit number by 2:  39 x 2 = 78
Add a decimal point between 7 and 8
Answer = 7.8

83 ÷ 5 = 16.6
Hack:
Multiply the 2 digit number by 2:  83 x 2 = 166
Add a decimal point between 16 and 6
Answer = 16.6

45  ÷ 5 = 9.0
Hack:
Multiply the 2 digit number by 2:  45 x 2 = 90
Add a decimal point between 9 and 0
Answer = 9.0  (or 9)