Question 11610
First, I don't see any need to convert Km to feet. Ok to convert 2 minutes to 120 seconds.

For the primary wave:

{{{d1 = (r1)t1}}}
{{{d1 = (80)t1}}}

For the secondary wave:

{{{d2 = (r2)t2}}}
{{{d2 = (5)(t1+120)}}}

But, d1 = d2, so:

{{{80t1 = 5(t1+120)}}} Simplify and solve for t1.
{{{80t1 = 5t1 + 600}}} Subtract 5t1 from both sides.
{{{75t1 = 600}}} Divide both sides by 75.
{{{t1 = 8}}}seconds.  Substitute this into {{{d1 = 80t1}}} and solve for d1.

{{{d1 = 80(8)}}}
{{{d1 = 640}}} Km.

Check:

{{{d2 = r2(t1+120)}}}
{{{d2 = 5(8+120)}}}
{{{d2 = 5(128)}}}
{{{d2 = 640}}}Km.