Explanation:
Given
SP = 210
profit percent (p%) = 5%
CP = X = ?
We know
Cost price (CP)

hope it helps :)❤
Or in another way
Let the cost price be x then
Profit amount
= 5 % of CP
= 5/100 * x
= x / 20
Selling Price (SP)
210 = CP + Profit amount
210 = x + x / 20
210 = ( 20x + x) / 20
210 * 20 = 21x
4200 = 21x
x = 4200 / 21
x = 200
Therefore CP = 200
Hope you understood.