Answer: 60

So average score of last (n-10) test = x+1

Sum of scores of last n-10 test = (n-10)*(x+1) = nx â€“ ( 10x – n + 10)

So total score of first 10 test = 10x â€“ n + 10

20*10 = 10x â€“ n + 10

10x â€“ n = 190Â —————1)

average score of first (n-10) test = x-1

Sum of scores of first n-10 test = (n-10)*(x-1) = nx â€“ ( n +10x â€“ 10)

So total score of first 10 test = ( n +10x â€“ 10)

30*10 = ( n +10x â€“ 10)

n + 10x = 310 —————2)

by solving eq 1 ) & eq 2)

n = 60, x = 25

so total number of test taken by him = 60

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