Word Problems Leading To Linear Inequalities

Words problems leading to inequalities are handled in a similar fashion like the linear equations except that the equality sign (=) changes to inequality sign.

Examples;

1. One-fifth of a number added to two-third of the same number is greater than 26. Find the range of values of the number.

Solution

Step 1

Let x represent the unknown number

The mathematical interpretation of the question will be;

^x/₅ + ^2x/₃ > 26

Step 2

Multiply through by 15 which is the L.C.M of 5 and 3;

3x + 10x > 390

13x > 390

Step 3

Divide both sides by 13;

x > ³⁹⁰/₁₃

x > 30.

Therefore, the range of values of x will be numbers greater than 30, which are 31, 32, 33, 34, 35 …

2. Ade bought x biros at #8 each and (x + 4) rulers at #20 each. He spent less than #200. Form an inequality for the statement and solve it.

Solution

Step 1

Interpret the statement;

The biros cost #8x …….(1)

The rulers cost #20(x +4) …….(2)

Step 2

Expand the second statement and you will have;

(#20x + #80)

Step 3

Combining (1) and (2) gives the total cost of both biros and rulers;

#(8x + 20x + 80) which is less than (<) #200.

Step 4

Forming an inequality equation below, you’ll have;

8x + 20x + 80 < 200

Step 5

Re-arrange and collect like terms;

Hence 28x < 200 – 80 (This is the inequality statement)

28x < 120

Step 6

Divide both sides by 28;

x < ¹²⁰/₂₈

Therefore, x < 4.29.

Practice these questions below;

1. The sum of twice a number and 5 is less than the sum of one-third of the number and 6.

2. Two-thirds of a certain number is greater than the sum of the number and 6.

3. The sum of twice a number and 15 is less than thrice the same number minus 9.

4. A cyclist travels xkm in 4 hours, then (x + 60)km in 7 hours. Its average speed does not exceed 150km/h.

 
5. A boy bought x mangoes at #5 and 3x oranges at #6. He collected some balance from #30.