L = liz's age now
J = Jack's age now
L-1 and J-1 are their ages one year ago
L-1=3(J-1)
L-1=3J-3
L=3J-3+1
L=3J-2 (to be used later)
L+2 and J+2 are their ages in two years
L+2=2(J+2)
replace L by 3J-2
3J-2+2=2(J+2)
3J=2J+4
3J-2J=4
J=4 is Jack's age now
L-1=3(J-1)
L-1=3(4-1)
L-1=3*3
L-1=9
L=9+1
L=10 is Liz's age now
