bile urna 1 = b1
bile urna 2 = b2
b1 + b2 = 300
b1 - 10 = ( b2 + 10 ) * 3
b1 - 10 = 3 * b2 + 30
b1 - 3 * b2 = 10 + 30
b1 - 3 * b2 = 40
b1 = 40 + 3 * b2
40 + 3 * b2 + b2 = 300
40 + 4 * b2 = 300
4 * b2 = 300 - 40
4 * b2 = 260
b2 = 260/4
b2 = 65
b1 = 40 + 3 * b2
b1 = 40 + 3 * 65
b1 = 40 + 195
b1 = 235
in prima urna au fost initial 235 de bile.