Question

John’s sister’s age is 10 years less than 5 times of John’s age and given that sister’s age is 7 years. Write the following statement in an equation.
(a) $10+5x=7$,
(b) $5x-10=7$,
(c) $10\times 5x=7$,
(d) $10=5x-7$.

Answer 1

Hint: We start solving the problem by assigning a variable to represent the age of John. We then multiply this variable with 5 in order to get five times the age of John. We subtract 10 from the obtained equation of the five times of age of John to get the age of John’s sister. We equation the final equation with the age given in the problem to get the desired final equation.

Complete step-by-step answer:
According to the problem, it is given that John’s sister’s age is 10 years less than 5 times of John’s age and the age of John’s sister is 7 years. We need to write the given statement in an equation.
Let us assume the age of John as x years. Let us multiply John’s age with 5 so that we get 5 times of John’s age. So, we need to multiply x with 5 in order to get the 5 times of John’s age. We get 5 times John's age as $5x$ years.
According to the problem, John's sister’s age is 10 years less than the 5 times of John's age. So, we need to subtract 10 from $5x$ in order to get 10 years less than the 5 times of the age of John. we get \[5x-10\] as the age of John’s sister.
According to the problem, the age of John’s sister is 7 years and we calculated the age of John’s sister as \[5x-10\]. So, we equate both values to get the given statement in an equation.
We have the given statement in an equation as $5x-10=7$.
The equation that tells about the given statement is $5x-10=7$.
The correct option for the given problem is (b).

Note: We can also find the age of john and then check the options accordingly to get the correct one. Whenever we get this type of problem, we need to start solving it by assigning a variable to unknown age. Similarly, we can expect problems to find the age of john, sum of age of john and his sister.