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Hi julle, you need to find (r+t)/r and are given something involving p, r, and t. So, let's first drive out p:
p/r=3/5 & p/t=9/10
Dividing the first ratio by the second,
(p/r)*(t/p)=(3/5)*(10/9)
=> t/r=2/3 (so, you need only +r in numerator. How about adding 1 to both sides?)
t/r+1=2/3+1
=> (t+r)/r=5/3. It's D.
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