Question 79474
Wow! this problem is either pretty tough or I'm just not reading it right.
Even if I'm off base, you might get something out of my method.
The thing to watch carefully is the words "now" and "when" and phrases
"will be" and "was".
Let M = Mutt's age now
Let J = Jeff's age now
The problem is saying that Mutt's age now is half of Jeff's age at 
some future time. I'll call that time {{{J + a}}} where a is in years
So now I've got {{{M = (J + a) / 2}}}
When Jeff is {{{J + a}}} years old, Mutt will be {{{M + a}}} years old
The problem says that Mutt's age then will be twice what Jeffs
age was at some time in the past. I can now say {{{M + a = 2(J - b)}}}
where b are the years to be subtracted from M and J.
At this time in the past, -b years, what was Mutts age? The problem says
it was {{{M - b = J / 2}}}, half as old as Jeff is now.
Going back,
{{{M = (J + a) / 2}}} What's a in this equation? To find it, solve
{{{M + a = 2(J - b)}}} for a.
{{{a = 2(J - b) - M}}}
substituting,
{{{M = (J + 2(J - b) - M) / 2}}}
{{{2M = J + 2J - 2b - M}}} 
{{{3M = 3J - 2b}}} What's b? Use  {{{M - b = J / 2}}} and solve for b.
{{{b = M - J/2}}}
{{{3M = 3J - 2(M - J/2)}}}
{{{3M = 3J - 2M + J}}}
{{{5M = 4J}}}
{{{M = 4J/5}}}
Now use the last piece of information
{{{J + M + 5 = 100}}}
{{{J + 4J/5 + 5 = 100}}}
{{{9J/5 = 95}}}
{{{J = 52.78}}} Jeff's age now
{{{M = 4J/5}}}
{{{M = 42.22}}} Mutt's age now
I plugged the answers back into the equations and they checked,
but I've been wrong before, so use your judgement.