implicit differentiation word problem

I am frustrated to death with this problem and would really appreciate some guidane.

Lynn and Charlie work in offices that are 40 miles apart. Lynn leaves home and drives to her office going due east at a rate of 50 mph. At the same time, charleie leaves home and drives to his office going due north at a rate of 60mph. How fast is the distance between Lynn and Charlie changing when Lynn has driven 10 miles.

The diagram in the problem shows that it's supposed to use pythagoran theorem. The people both leave fromo practically the same spot so it makes a right angle triangle.

Thanks very much