19. The present age of brother and sister are in the ration 4:5 Before three years the ration of their age was 3:4. find their present age-

Sister = 15 years old

Brother = 12 years old

Step-by-step explanation:

Right now, if the brother is B years old and the sister is S years old, B = (4/5)S because the ratio from brother to sister is 4:5, so if one unit is X, B = 4X and S = 5X

Three years ago, the brother was B-3 years old and the sister was S-3 years old. Keeping one unit as X, we have

B = 3X

S = 4X

B-3 = (3/4)(S-3)

Therefore, we have a system of equations

B = (4/5)S

B-3 = (3/4)(S-3)

Substitute (4/5)S for B into the second equation to only have one variable

(4/5)S - 3 = (3/4)(S-3)

(4/5)S - 3 = (3/4)S - 9/4

add 3 to both sides and subtract (3/4)S from both sides to isolate the variable and its coefficient

(1/20)S = 3/4

multiply both sides by 20 to isolate the S

S = 15

B = (4/5)S = 12

answer: its the third option.

step-by-step explanation:

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