In certain ranges of a piano keyboard more than one string

Question

I need proficient help in the following problem:

Problem: In certain ranges of a piano keyboard, more than one string is tuned to the same note to provide extra loudness. For example, the note at 1.10 x 10² Hz has two strings at this frequency. If one string slips from its normal tension of 6.00 x 10² N to 5.40 x 10² N, what beat frequency is heard when the hammer strikes the two strings simultaneously?

Please show how you obtained the answer.