We need exactly 70p worth of coins. So that is one 50p coin and one 20p coin. Best way to do this is to draw a tree diagram.
We can only take the dashed paths shown above.
1st dashed path gives us = 2/7 x 5/6 = 10/42 = 5/21
2nd dashed path gives us = 5/7 x 2/6 = 10/42 = 5/21
Total = 5/21 + 5/21 = 10/21
So the answer is C.