First ratio is 7 : 3

Total = 10 units

Second ratio is 4 : 3

Total = 7 units

Total number of students didn't change. So the total number of units for both cases should be the same.

Make the total number of units the same.

10 units x 7 = 70 units
7 units x 10 = 70 units

Multiply first ratio by 7,

7 : 3 = 49 : 21

Multiply second ratio by 10,

4 : 3 = 40 : 30

49 - 40 = 9
21 + 9 = 30

So 9 units of students moved to Tampines from Bedok.

9 units = 108 students

1 unit = 108 students ÷ 9 = 12 students

Number of students who lived on Tampines in the end

= 30 units
= 12 students x 30
= 360 students

Pls advise next step. Thanks

Completed answer has been typed out. Check the current working above.

Thank you very much for ur help and explanation.

You're welcome

Can I ask u another question?

Sure

in the end
T: 3u
B: 4u

at first
T: 3u - 108
B: 4u + 108
ratio = B : T = 7 : 3

3 x (4u + 108) = 7 x (3u - 108)
12u + 3x108 = 21u - 7x108
u = 10x108/(21-12) = 120

in the end,
T has 3x120 = 360 students


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Thank you