Sorry to be a complete geek but it's actually 2.85 syns for 25ml (4 syns for 35ml - 4 / 35 x 25 = 2.85). Small difference but this came up in class last week since a woman was having 2 doubles each night but she was counting it as 70 syns for the week (2.5 x 4 x 7) but it was actually 80 syns for the week so she ended up going over.
No one was very happy about me pointing this one out!