Help!我有條數學題唔識 (F4)

Assuming the first digit is A and the second is B,

10A+B=3(A+B)..............(1)
10A+B+9=4(A+B)..............(2)

From (1),

7A=2B
B=3.5A.......................(3)

From (2),

6A+9=3B...................(4)

Inserting (3) into (4),

6A+9=3(3.5A)
A=2

Inserting A=2 into (3),

B=3.5(2)
B=7

Please understand that 97= 9*10+7 and a number ab is equal to
a*10+b



Let a be the ten digit number, Let b be the unit digit number

(From the first sentence, we know that:)

3*(a+b) = 10a+b

3a+3b = 10a+b

7a = 2b...................................................(1)

(From the second sentence, we know that:)

4*(a+b) = 10a+b+9

4a+4b = 10a+b+9

3b = 6a+9

b= 2a+3.....................................................(2)

Sub (2) into (1)

7a = 2*(2a+3)

7a = 4a+6

3a = 6

a = 2

Sub a=2 into (2)

b = 2(2)+3

b = 7