When dealing with ratios it's helpful to convert to fractions. The ratio 5:7 is to be converted into 5+7=12 units. The wood is divided into 5 of these units and 7 of the units. So that's 5/12 and 7/12. The length of the wood is 60cm, and that's equivalent to 12 units, so a unit is 60/12=5 cm. therefore 5 units is 25 cm.