Question

What would be the final temperature of mixture of 50g of water at 20C temperature and 50g of water at 40C temperature?

Answer 1

50 gms water at 20 ⁰ C and 50 gms of water at 40⁰ C are mixed.  Since the masses of the liquid at different temperatures are same, the answer is very easy and simple :  average of 20⁰C and 40⁰C.  that is:  30⁰C.
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final temperature of the mixture =
     = [ m1 * T1 + m2 * T2 ] / (m1 + m2)
     = [ 50 gms * 20⁰ C + 50 gms * 40⁰C ]  / (50+50)

     = 3,000 / 100 = 30⁰C

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another way using specific heats :

   let the final temperature be = T ⁰C
   Amount of heat given out by the hot water = m * s * (40⁰C - T)
           = 50 gms * s* (40 -T)
   Amount of heat taken in by the cold water = m * s * (T - 20⁰C)
       = 50 gms * s * (T - 20 )

   As the amounts are equal, because the heat is transferred from hotwater to the cold water without any loss of heat to any surroundings,
           50 * s * (40 -T) = 50 gm * s  * (T-20)
                 40 - T = T - 20
             2 T = 60    => T = 30⁰C
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Answer 2

T = M1T1+M2T2/M1+M2

given, M1 : 50g. T1 : 20C

M2 : 50g. T2 : 40C

final temperature: M1T1+M2T2/M1+M2

: (50*20+50*40) /50+50

:. (3000)/100

: 30C