Jack is twice as old as Jill ,but in five years time he will only be one and a half as old as Jill.

这句话的意思是,jack的岁数是jill的两倍,但是5年后jack的岁数将是jill的1.5倍.
所以可以设jack的岁数为x,jill的岁数为y,那么
x=2y
x+5=1.5×（y+5）
解这两个方程可得x=10,y=5.
所以jack现在10岁,jill现在5岁,带回题目中验证,正好符合要求.