Emma is 10 years old. she asked her father how old he is. her father answered, "when you are my age I will be 70." how old is Emma's father.

40

Step-by-step explanation:

Emma age: yE = 10 + x   (x is timeline)

Father age: yF = A + x (x is timeline, A is Father age today)

in time x1: Emma will be A years old, and Father will be 70 years old.

yE (x1) = 10 + x1 = A

yF (x1) = A + x1 = 70

System of equations with unknown values: x1 and A.

substract equation 1 - equation 2:

10 - A = A - 70 ⇒ 80 = 2A ⇒ A = 40

So, father is 40 years old now