Afisherman caught 15kg fishnon monday and 23kg on tuesday. on tuesday there were twice as many fish as on monday, but their average mass was, 1|8 kg less. how many fish did the fisherman catch on monday?

28 fish were caught on Monday. 
 Let's create some equations for this problem. The following variables will be used
 Fm = Fish caught on Monday
 Mm = Average mass of fish caught on Monday
 Ft = Fish caught on Tuesday
 Mt = Average mass of fish caught on Tuesday
  So we have the following equations:
 15 = Fm * Mm
 23 = Ft * Mt
 Ft = 2 * Fm
 Mt = Mm - 0.125 
 Let's substitute the equations for Ft and Mt into the equation for fish caught on Tuesday
 23 = Ft * Mt
 23 = 2 * Fm(Mm - 0.125)
 23 = 2 * Fm * Mm - 2 * Fm *0.125
 23 = 2 * Fm * Mm - 0.25 * Fm 
 Now we know from Monday that Fm*Mm is 15, so let's substitute that into the equation and solve for Fm
 23 = 2 * Fm * Mm - 0.25 * Fm
 23 = 2 * 15 - 0.25 * Fm
 23 = 30 - 0.25 * Fm
 -7 = - 0.25 * Fm
 28 = Fm
 
 We now know that 28 fish were caught on Monday. 
 Let's verify that everything makes sense.
 Fm = 28
 Mm = 15 / 28
 Ft = 2 * Fm = 2 * 28 = 56
 Mt = 23 / 56 
 Mm - Mt = 1/8
 15/28 - 23/56 = 1/8
 30/56 - 23/56 = 1/8
 7/56 = 1/8
 1/8 = 1/8
 And we get a true statement. So the calculated number of fish caught on Monday is correct.