如何计算市盈率变动带来的收益变化率？

　原书中有这样一段话：
For example, an initial dividend yield of, say, 3 percent plus a forecasted earnings growth of 7 percent annually over the next 10 years would bring the return to 10 percent. A change in the price–earnings ratio — from, say, 15 times at the beginning of the period to a forecasted 18 times at the end — would add 2 percentage points to that total, bringing the return on stocks to 12 percent.
初始红利收入为3%，预期收益增长率为每年7%，因此基础收益率为10%（即3%+7%），市盈率由15变为18后会使收益率再增加2%。
这个2%是怎么计算来的？实在搞不懂，求高手指点！