Question 2325
define:

x = grandpa's age now
y = grandma's age now.

1. {{{(x+y)/2 = 70}}} 
{{{x+y=140}}}--eqn1

2. when he was her age: he was y, she was y-(x-y) where (x-y) is the difference in their ages. Use numbers to check this..just make some up and look at how you would work it out with numbers.

x = 2(y-(x-y)) --eqn2. this is a tricky one to get.

Re-arranging gives...

x = 2(2y-x)
x = 4y - 2x
3x = 4y --eqn3

sub 1 into 3 for x, gives

3(140-y) = 4y
420-3y= 4y
7y=420
y=60

so x=80

check...grandma was 40 when he was her age now, 60. And Grandpa's current age of 80 is twice that..so correct.

jon.