Question 270192
The equation we need for this is
(1/f)(x) + (1/s)(x) = 1
where 
f = father time
s = son time
x = time together
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we know that x = 30, f = 40. We get
(i) {{{(1/40)*30 + (1/s)*30 = 1}}}
step 1 - 
multiply by the common denominator of 40s to get
(ii) {{{30s + 1200 = 40s}}}
step 2- subtract s to get
(iii) {{{1200 = 10s}}}
step 3 - divide to get
(iv) {{{s = 120}}}