help please

Hmm, let me try this one.

We know that a:b=9:4, and 10b=7c, so that means b:c=10:7

a:b=9:4

c:b=7:10   Find the LCM of 4 and 10, which is 20.

So, a:b=45:20

c:b=14:20

Thus, the answer is 45:14

Ok, Tertre fixed his error, and his answer looks right now!

is  this correct?

10b=7c, b=7/10c

So, 4x=7/10c, x=7/10/4=7/10*1/4=7/40c

9x=7/40*9=63/40c.

a=63/40c

Wait, so is it 63:40?