Jack’s average score in the last 8 tests is 70. What should be his score in his next test, so that his average score will be 72?

\$"Average" = "sum of set of numbers" / "amount of numbers in set"\$

The average score of 8 tests = 70

Sum of scores in \$(8 "tests")/8 = 70\$

Multiply both sides by 8.

⇒ \$\cancel8^1 \times (8 "tests")/\cancel8^1 = 70 \times 8\$

Sum of scores in 8 tests = 560

Let \$x\$ be his score in his next test.

Average score of 9 tests = 72.

\$("Sum of scores in 8 tests" + x)/9 = 72\$

⇒ \$(560 + x)/9 = 72\$

Multiply both sides by 9.

⇒ \$\cancel9^1 \times (560 + x)/\cancel9^1 = 72 \times 9\$

⇒ \$560 + x = 648\$

Subtract 560 from both sides.

⇒ \$\cancel560 + x - \cancel560 = 648 - 560\$

⇒ \$x = 88\$

So, Jack score in the next test should be 88.