x = age of julia
2x = age of Wendy
10x = age of dad (because dad is five times as old as Wendy)
(x+4)*6=10x+4 (because in four years, that's why I added 4 to both sides, dad's age will be now 10x+4 and Julia's age x+4, now dad is 6 times as old and I multiplied Julia's age by six) ==> 6x+24=10x+4, now subtract 4 from both sides:
6x+20=10x, let's subtract 6x from both sides
20=4x ==> x=5
because age of father is 10x, we substitute x value and we get
10*5=50
