The answer is 573
S is the three digit number.
h - a number of hundreds
t - a number of tens
o - a number of ones
S = 100h + 10t + o
The last two digits add to make ten: t + o = 10
The first and last digits add to make eight: h + o = 8
The first two digits add to make twelve: h + t = 12
t + o = 10 ⇒ o = 10 - t
h + o = 8 ⇒ o = 8 - h
_______
10 - t = 8 - h
h + t = 12 ⇒ h = 12 - t
_______
10 - t = 8 - (12 - t)
10 - t = 8 - 12 + t
10 - t = -4 + t
10 + 4 = t + t
2t = 14
t = 14/2 = 7
______
h = 12 - t = 12 - 7 = 5
______
o = 8 - h = 8 - 5 = 3
S = 100h + 10t + o
h = 5
t = 7
o = 3
S = 100*5 + 10*7 + 3
= 500 + 70 + 3
= 573
