4) Ahmad is twice as old as Bobby. John is 7 years younger than Ahmad. If the sum of their age is 38, how old are the three boys?

Let A = Ahmad's age      B  = Bobby's age     J = John's age

1)  A = 2*B  or B = A/2

2) J = A - 7

3) A + B + J = 38

You have 3 equations and 3 unknowns. Start by substituting equation 1) and equation 2) into equation 3) and solve for A:

A + A/2 + (A-7) = 38        (combine like terms)

2.5*A = 38 + 7 = 45

A = 45/2.5 = 18 yrs old (Ahmad's age)

B = A/2 = 18/2 = 9 yrs old (Bobby's age)

J = A - 7 = 18 - 7 = 11 yrs old (John's age)

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