First question.
#3,500,000 = (3,500,000/420)Euro = 25000/3 Euro <-- here we use exact value for a more exact answer.
25000/3 Euro = 25000/(3*0.00020509) Kringles = 40632567.8157556844962374 Kringles
which is about 40 million kringles or 0.406 tbc
Second question.
Get yourself an easier case first.
Now if kringle = 1 Euro, then kringle = 1 * 420 * naira.
And if kringle = 2 Euro, then kringle = 1/2 * 420 * naira.
So if kringle = 0.00020509 Euro, then kringle = 420/0.00020509 naira.