Question 148616
You need to find the prime factors in each denominator value.
3a= 3 * a
24b = 2*2*2*3 * b
7a = 7 * a
25b = 5*5*b

Now look at each line above and find the lines that contain the 'most' factors of the same type.

3a has one 3 and one a. So we need 3 and a
24b has three 2's , one 3 and one b. We need the three 2's and the b. We already have one 3 from the previous entry.
7a has one 7 and one a. We already have one a, We need the one 7.
25b has two 5's and one b. We have one b already. We need both 5's since we don't have any of them yet.

So let's collect all the factors we 'kept' above.
{{{3*a * 2*2*2 *b  *7 * 5*5}}}
So all we need to do now is multiply it all together to get
{{{3*8*7*25 * a*b}}}
{{{4200*ab}}}
{{{4200ab}}}