Question 734376
n hours just the new sweeper alone
<pre>
                RATE        TIME          JOB

OLD             1/30         30            1

NEW             1/n           n            1

BOTH           1/7.5          7.5          1
</pre>
Rates of the two working together are simply additive.
{{{highlight(1/30+1/n=1/7.5)}}}
Solve.