Answer: 92 or 119
in 4 months, 2 months with 30 days per month, 2 months with 31 days per month
suppose: the number to find is X
have: 90 < X < 30.2 + 31. 2 ⇔ 90 < X < 122
if X is a number has 2 digits => X is xy
=> [tex]\left \{ {{x+y=11} \atop {90<x\leq 100}} \right.[/tex]
=> xy = 92
if X is a number has 3 digits => X is abc
we have: [tex]\left \{ {{a+b+c=11} \atop {100\leq abc<122}} \right.[/tex]
because 100 ≤ abc < 122 => a = 1; b = 0 or 1
+) if a =1 ,b = 0 but a+ b +c = 11 => c = 10 (removed because c is a 1-digit number)
+) if a = 1, b = 1 => c = 9
=> abc = 119
=> X is 92 or 119
Step-by-step explanation:
