There is a French film called 'The City of Lost Children' in which the character of the professor has a brain which he keeps in a vat and he uses this as his companion.

The types of VAT........

1 ) INPUT VAT @ 4 %

2 ) INPUT VAT @ 1 %

3 ) INPUT VAT @ 12.5 %

4 ) OUTPUT VAT @ 1 %

5 ) OUTPUT VAT @ 4 %

6 ) OUTPUT VAT @ 12.5 %

If the VAT rate is V% then the cost price with VAT is = Cost Price*(1 + v/100)

The inlet valve fills 1/6th of the vat in an hour.
The outlet valve drains 1/10th of the vat in an hour.

When they are both open, (1/6th - 1/10th) of the vat fills in an hour.

(1/6 - 1/10) = (5/30 - 3/30) = 2/30 = 1/15th fills in one hour.

So with both valves open, it takes 15 hours to fill the vat.