UPSEE Solved Model Paper 3

Apatite is a mineral that is found in tooth enamel. When fluoride toothpastes are used, this mineral is converted to fluoroapatite, which is more resistant to tooth decay. The formula of apatite is Ca5(PO4)xF. What is the value of x if the compound contains 18.45% by weight of phosphorus? (Atomic weight of P=31, Ca=40, F=19 and O=16)