Question 344587
Let the 1st, 2nd, 3rd, 4th, and 5th digits be F, S, T, H, and I, respectively
Then S = S
T = F + S
I = 2(F + S) + S___I = 2F + 2S + S___I = 2F + 3S
F = 2S
H = 4F   


Now, since 1st digit or F = 2S, we can say that:
T = F + S_____T  = 2S + S, or 3S
H = 4F____H = 4(2S)____H = 8S
I = 2F + 3S____I = 2(2S) + 3S___I = 4S + 3S____I = 7S


Now we can see that:
F = 2S
S = S
T = 3S
H = 8S
I = 7S


We can also see that the largest digit is the 4th digit as H = 8S
Now, it’s clear that S or the 2nd digit CANNOT be any digit from 2 – 9 as this would yield a 2-digit number (e.g. 8 * 2 = 16)


This means that S or the 2nd digit has to be less than 2, and since it’s given that it CANNOT be 0, then S must equal 1


Therefore:
F = 2S = 2(1) = 2
S = S = 1
T = 3S = 3(1) = 3
H = 8S = 8(1) = 8
I = 7S = 7(1) = 7


This makes the 5-digit number: {{{highlight_green(21387)}}}


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