accrual and prepayment

A company’s telephone bill consists of two elements. One is a quarterly rental charge, payable in advance; the other is a quarterly charge for calls made, payable in arrears. At 1 April 20×9 the previous bill dated 1March 20×9 had included line rental of $90. Estimated call charges during March 20×9 were $80.
During the following 12 months, bills totalling $2,145 were received on 1 June, 1 september, 1 December 20X9 and 1 March 20Y0, each containing rental of $90 as well as call charges. Estimated call charges for March 20YO were $120. What is the amount to be charged to the statement of profit or loss for the year ended 31 March 20Y0?
A $2,185
B $2,205
C $2,155
D $2,215
The answer is A $2,185. Prepayment b/f $60(2/3 x 90) +$2,145 -prepayment c/f $60 -accrual b/f $80 + accrual c/f $120= $2,185

1- i don’t understand that why prepayment b/f is 60$(ie why we multiply 90×(2/3) ) & why prepayment c/f is 60 as well?

2- is 2145$ added to prepayment b/f as additional prepayment ? or as cash paid & why ?

thank you.