Using trial and error and confirmation method:
every 7 tables there are 25 chairs.

If each 3 tables has 5 chairs, then 4 tables has 3 chairs, then
3*5 + 4*3 = 27 not equal to 25

similarly, try
2*5chairs + 5*3chairs = 25 chairs

You get 2 : 5

Using the calculation method:
'a' tables has 3 chairs
'b' tables has 5 chairs
total no of chairs = 3a + 5b
total no of tables = a + b
a + b = 7
3a + 3b = 21
3a + 5b= 25
2b = 4
b = 2
a = 7-2 = 5
total no of chairs = 3chairs*(5 tables) + 5chairs*(2 tables)
therefore ratio of tables is 2 : 5