This can be solved easily using algorithm,

x + y = 30 ---(i)

50x +100y=1800

=> x+2y=36 ---- (ii)

Therefore from equation (i) and (ii) we get, y=6

Let, we can use the algorithm to find how 50 and 100 make the average to 80

................50......................100

............................80

...........(100-80)...............(80-50)

..............=20......................=30

>> ............2 .........................3 (this is the ratio by which 50 & 100 are to be combined to make average of 80)

>> .............4.........................6 (just multiplying both sides with 2, it does not change the ratio)

This means if we have six 100 and four 50 we will get average 80 considering all notes

So, extra 50 notes are 24-4= 20

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