Answer:
8/25
Step-by-step explanation:
8/15 + x = 21/25
x = 21/25 - 8/15
now we need to bring both fractions to the same denominator (bottom part), so that we can do the subtraction.
what is the least common multiple (LCM) of 25 and 15 ? that will be the common denominator.
I usually use the prime factor factorization of the involved numbers.
15 : 3, 5 (15 = 3×5, there are no smaller prime factors)
25 : 5, 5 (25 = 5×5, there are no smaller prime factors)
the LCM is the combination of the longest chains of each prime factor :
3×5×5 = 75
so, we being both fractions to 75 denominators.
how many 1/75th are 1/25 ? as 25×3 = 75, we have 3/3 × 1/25 = 3/75 (= 1/25).
so,
21/25 = 21×3/75 = 63/75
how many 1/75th are 1/15 ? as 15×5 = 75, we have 5/5 × 1/15 = 5/75 (= 1/15).
so,
8/15 = 8×5/75 = 40/75
remember, we have to multiply the numerator and the denominator by the same value to keep the original value of the fraction unchanged.
and now our equation looks like
x = 64/75 - 40/75 = 24/75 = 8/25 (as we could divide to and bottom by 3).
