Explanation:
1.
200 - 7/21 = (200×21 - 7)/21 = (4200 - 7)/21 =
= 4193/21 = 599/3 = (199×3 + 2)/3 =
= 199 2/3
we could have seen that 7/21 = 1/3.
so, then this problem was
200 - 1/3 = (200×3 - 1)/3 = 599/3 = 199 2/3
(see above)
2.
50 - 4/7 = (50×7 - 4)/7 = (350 - 4)/7 =
= 346/7 = (49×7 + 3)/7 = 49 3/7
3.
24 1/4 - 20 3/5 = (24×4 + 1)/4 - (20×5 + 3)/5 =
= 97/4 - 103/5
to calculate this we need to bring both fractions to the save denominator. the LCM (lowest common multiple) of 4 and 5 is 20.
so,
97/4 - 103/5 = (97×5)/(4×5) - (103×4)/(5×4) =
= 485/20 - 412/20 = 73/20 =
= (3×20 + 13)/20 = 3 13/20
4.
10 5/8 - 6 4/9 = (10×8 + 5)/8 - (6×9 + 4)/9 =
= 85/8 - 58/9
the LCM of 8 and 9 is 72.
85/8 - 58/9 = (85×9)/(8×9) - (58×8)/(9×8) =
= 765/72 - 464/72 = 301/72 =
= (4×72 + 13)/72 = 4 13/72
5.
6 1/2 - 5 3/4 = (6×2 + 1)/2 - (5×4 + 3)/4 =
= 13/2 - 23/4
the LCM of 2 and 4 is 4.
13/2 - 23/4 = (13×2)/(2×2) - 23/4 =
= 26/4 - 23/4 = 3/4