There are 450 seats in the lower level of a concert hall with b balcony seats in upper level so far 170 tickets have been sold which Is 1/5 of the total number of seat in concert hall how many tickets sold are balcony seats

There are 400 balcony seats.

Let b be the number of balcony seats.  There are 450 lower seats; this makes the total number of seats 450+b.  We know that there were 170 tickets sold, and that this is 1/5 the total number of seats; therefore our equation is
1/5(450+b)=170

Use the distributive property with the 1/5:
1/5*450 + 1/5*b = 170
90 + 1/5b = 170

Subtract 90 from both sides:
90 + 1/5b - 90 = 170 - 90
1/5b = 80

We can also write 1/5b as b/5:
b/5 = 80

Multiply both sides by 5:
(b/5) * 5 = 80 * 5
b = 400