10.5 hours
let t be time then distance travelled by both is
Mr Burges ⇔ d = 50t ( distance = speed × time )
Mrs Burges ⇒ d = 70(t - 3)
At the point of overtaking both will have travelled the same distance
equating their distance and solving for t
70(t - 3) = 50t
70t - 210 = 50t ( subtract 50t from both sides )
20t - 210 = 0 ( add 210 to both sides )
20t = 210 ( divide both sides by 20 )
t = 10.5 hours
