Mary, who is sixteen years old, is four times as old as her brother. How old will Mary be when she is twice as old as her brother?

Answer 1

M=16
M=4B
(M+t)=2(B+t)

so B= 4, then
M+t=8+2t
8=t

so mary will be 16+8=24, brother 12, now Mary is 16, brother is 4