let

t1 = the time of the experienced accountant in hours

v1 = 1/t1 books per hour the rate of the experienced accountant

t2 = the time of the new accountant in hours

v2 = 1/t2 books per hour the rate of new accountant

an experienced accountant can balance the books twice as fast as a new accountant

v1 = 2v2

1/t1 = 2(1/t2)

(1/t2) = 1/(2t1)

together it takes them 4 hours

1/t1 + 1/t2 = 1/4

plug (1/t2) = 1/(2t1) into the last equation

1/t1 + 1/(2t1) = 1/4

by solving we find

t1 = 6 hours

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the experienced accountant working alone will need 6 hours to balance the books.