Hi broker,
In general, questions that deal with very large numbers are best dealt with by breaking your numbers into prime factors.
When a question gives you an equation with a variable somewhere in the exponents, the best approach is typically to manipulate the two sides of the equation to make them look the same, so that you can be sure that the exponents equal each other. This is what Rahul did.
An even quicker alternative is to notice that because both sides are equal, they must have the same number of 5's if you wrote all the prime factors in a string. Since the left side has 21 fives (5^21), the right side must have 21 fives. Each 10 has a 5^1 (10=5*2), so to have 21 fives on the right, we must have 21 tens. 10^n must be 2^21 5^21 and n=21.
A take-away lesson and a step by step video solution can be found at
GMATPrep Question 1001. If you have trouble with similar questions, set topic='Roots & Exponents' and difficulty='400-500'
-Patrick
