Explanation:
he has to pay £20 no matter what. even if he does not use his phone in that month at all.
and that includes 100 minutes phone calls and 200 texts.
everything beyond that he has to pay extra.
with 140 minutes of phone calls he has 140-100 = 40 extra calling minutes.
with 261 texts he has 261-200 = 61 extra texts.
that costs him
40×9 + 61×11 = 1031 p
extra (in addition to the £20).
as there are 100 pence (p) in one pound (£),
1031 p = 1031/100 = £10.31
so, his total charge for that month is
20 + 10.31 = £30.31