First of all, 100cm makes 1m. So compare the statements 100cm=1m 2.5cm=Y I represented the unknown with Y. Cross and multiply. 2.5 will multiply 1 to give 2.5 and Y will multiply 100 to give 100Y. That will be; 100Y=2.5 Divide both sides by 100 Y=2.5/100 But remember; 100cm is 1 meter. So that will be the same as; Y=2.5cm/1m which is C Hope that helped. Have a nice day