Answer:

210 cm

Step-by-step explanation:

If you need a simple solution do this:

1 book = 6cm

therefore, 35 x 6 = 210

but if it's a bit complicated or particularly connected with ratio then use this:

1 book = 6cm

25 books = __cm

therefore, 1 : 6 25 : __

x = 6(25)

x = 220

Note:

You maybe wondering where 1 is, it is in front of x. As we all know in algebra one is often not written.