Question 278519
let m = mikes age, d = dans age.

from the first statement : m+d = 20   so   d = 20-m

from the second statement  (3/4)(m+4) = d+4
working a bit on the second statement gives :

3(m+4) = 4(d+4)
3m + 12 = 4d + 16
3m = 4d + 4

substituting from the first statement :

3m = 4(20-m) + 4

3m = 80 - 4m + 4
7m = 84
m = 12

So Mike is 12 and Dan is 8