The texi firm charges 3:00 standing charge, plus 50p per mile. how much will it cost Jenny to travel 8 miles

Assuming you meant:

The taxi firm charges £3.00 standing charge, plus 50p per mile. How much will it cost Jenny to travel 8 miles?

Then the answer would be,

50p (per mile) x 8 (miles) = £4.00.

£4.00 + £3.00 (S. O) = £7.00.

Therefore, the whole journey will cost Jenny £7.00.

-Hope this is what you were looking for, if not sorry.